I have lived in London for more than 6 years now and it is a city I feel very at home in. It is probably also one of the most visited cities in the world, so recommending some things which people do not know, might be quite a task. I will simply try to describe the things which I still like doing after such a long time living here. You might be very familiar with a few things, but I will do my best to put a twist on it.
So if it is your first time in London definitely make sure to simply check Buckingham Palace, Westminster, London Eye, Trafalgar Square etc off the list. All of which you should be able to do in half a day or a day combined.
1. Southbank
Go for a very relaxing stroll along Southbank. Ideally you access the south side of the river from Embankment (north side) via Jubilee Bridge. Do so via the western bridge so you can see Westminster, London Eye and Vauxhall in the distance. Do this because once you have crossed the bridge you will leave all this behind, because you most likely will have seen it already and the area around London Eye will be too packed. So once you crossed the bridge, head east. Southbank is full of culture, people, events, restaurants and bars. It most likely will be very busy here as well, but there are a few hacks to be in the middle of the action and still avoid the touristy crowds.
Right at the start is Royal Festival Hall which a cultural hub for concerts and events. The trick is to go into the back of the ground floor, grab some drinks in plastic cups and then take the big elevators to the 5th floor. There you will find a balcony, which looks right down on Southbank. Tourists usually don’t go here. It has everything you need: drinks, amazing view and chilled atmosphere. Don’t miss it.
Look down and right from that balcony and you can see another alternative chillout place in Southbank (at least in the summer) hidden in plain sight: Queen Elizabeth Roof Garden Bar. A lovely outdoor garden with a bar. Unlike the balcony of Royal Festival Hall in can get quite crowded, so make sure to arrive early enough or on a weekday. Same as above. Chilled out vibes and nice Southbank views.
If you keep heading East you will cross under a bridge with a book market which is quite popular. Under the bridge you will find BFI (British Film Institute). You will think, why would you want to go to a cinema. Even if you are not the biggest film buff, check out the venue. There is a newly furbished restaurant bar in the front (with seatings outside and a slightly hidden upstairs and outside bar). If you keep walking straight inside go until you reach the main screen and then turn right and walk all the way around it to the end of the building. There you will not only find a library, free viewing booths and a pretty cool movie shop, but also another restaurant with a pretty stylish cafe bar, with plenty of cozy seating opportunities. Again not a lot of tourists go the extra few steps to reach this place, but it is definitely worth it, especially when the weather is not so nice or in winter.
Once outside again keep going east and the next (slightly ugly concrete) building is the famous National Theatre. Again, you don’t need to be a theatre buff to enjoy the bar and outside terrace. Secluded space right in the middle of Southbank. Also a good spot to impress on a date.
There is plenty to discover in Southbank. Don’t just stay by the river (although the view there is pretty amazing), venture beyond the first tourist layer and go a bit further in and you will be surprised you will find. Like the nice restaurant bar Topolski, just a few minutes away.
2. Tate Modern
The Tate Modern an extremely famous modern art gallery (keep walking 15 minutes east from Southbank), which kinda is a must see even if you are not into modern art. The gallery is mostly free and merely the gigantic entrance hall will blow your mind. You probably could spend a full day here looking at the exhibitions. The twist: the relatively newly added top floor of the Blavatnik Building offers a free and stunning 360 degrees view on the city. Take the the elevator to the 10th floor. Another twist: Tate Late is an after hours drink, dance, culture event usually every last Friday of the month (check the website to confirm). There are often other events in and around Tate Modern, so swing by and see what’s going on.
3. St Paul’s Cathedral
Again, extremely famous and hard to miss or not know about, St Paul’s Cathedral is one of the most iconic cathedrals in the world. Why is it on this list? Because a lot of people think they can miss out on this gem or think it is too expensive. It surely has quite a steep price, but every penny is worth it (also, if you work in London or the UK (?) you pay once and can go for free for the rest of the year). Before you go in make sure that the Golden Gallery is open for visitors. While the inside of the main cathedral is already magnificently stunning for me the real joy is to climb up all the way to the top (often people don’t know they can). Start climbing the stairs from the inside until you reach the Whispering Gallery (here you can look down the to nave and find people whispering something into the walls which is supposed to be heard on the other side of the gallery by their friends. For some reason it never worked for me, but give it a try). Don’t stay too long because from there you want to make your way up the Stone Gallery, which is the bottom outside area of the dome. You already have amazing sights from here, but why stop when it gets even better. Again find the stairs for your way up to the marvellous Golden Gallery. The way up is quite steep and narrow, but the views from top will be unlike anything you have ever seen. Quite high up in the middle of the City of London, you have marvellous views. Extra tip: On a nice day, try to make it for sunset! You’re welcome! 
4. Primrose Hill
One of my favourite spots in London and one of the many on site shooting locations for Sense8. If you spend half a day in Regent’s Park, don’t forget to venture further north to reach Primrose Hill (behind the zoo). It is a little extra bit of park with a hill overlooking the city. When it comes to free views it doesn’t get much better than this. Come early or late (the night views are amazing too) to avoid many tourists. Or join them in sitting on the hill and picnicking and chilling out. Despite the tourists it attracts it feels a bit spiritual. Don’t forget to bring your drinks, chocolate, cheese, music or whatever you need to contemplate life.
5. Boat Tour to Greenwich (and the DLR back)
Go to Westminster, find the river boat pier and book a one-way ticket to Greenwich on a boat with an open upper deck. If you don’t want to buy drinks and snack on the boat, bring your own. The ride is about 40 minutes long, but you won’t regret it especially on a nice day it will give you the best view of the city in a short time. You will see all the major sites of the city from the comfort of sitting on the boat on the river Thames. There is nothing quite like it. It will give you the feeling to be in the middle of everything and at the same time you have the breeze around your head and can relax while beautiful London passes by you. Don’t forget to turn your head though in all directions as there is plenty to see. Once in Greenwich find your way to Greenwich park and walk up to the Royal Greenwich Observatory. Again amazing, but very different views on Canary Wharf and the City of London in the back. Get your drinks out relax and if you want to spend a little money you can go into the Observatory and stand on the famous 0 degrees meridian (although technically you can also do that from the outside, if you imagine where it continues). On the way back take the DLR (Dockland’s Light Railway, which is London’s automated mostly overground tram) back to the city (most likely Bank). The trick here is to get a seat at the very very front of the train (walk all the way to the right end of the platform inside Greenwich station to make sure you got the best option to do so) to have the most gorgeous view when driving back through Canary Wharf. It feels a bit like being transported to the future. Don’t miss it.
6. Kinky Kabaret
Don’t get fooled by the name it is neither kinky (much) nor (traditional) cabaret. Head to Soho on Monday late night and find the marvellous FREE Kinky Kabaret at Freedom Bar. Technically it is a gay venue, but it has always a very mixed crowd. Especially on Monday nights from 11pm-1am. You will see up and coming musical stars from the surrounding Westend shows performing live on a rather small stage, singing show tunes and pop songs. It is all about their big voices on that small stage and the absolutely stunning positive atmosphere. You can book a seat in advance or just hang out by the bar (which will give you more space to groove to the songs.) It is quite difficult to describe, but if you are in for magnificent free entertainment, it does not get any better than this. After the show it turns into a regular club. It is usually very crowded but not as packed that you cannot move like it will be on Fridays or Saturdays. If you don’t have many plans on a Tuesday morning, check it out!
So those are just a few things I like to do whenever someone visits me in this amazing city. There is so much to see and do, it really never stops.