Oh yeah, there’s a lot. Not so much in central London but certainly in the south and east. My neighbourhood in South London is riddled with art - the council apparently helps match up artists with people with blank walls.
But there’s no cool line on the ground there, so the original wins. 
There used to be a green laser shining north from Greenwich Observatory along the meridian. As I hear tell, they had to turn it off because it ended up shining into someone’s kitchen in a north London highrise.
This isn’t necessarily a “must do”, but it is quite a curiosity. The London Transport Museum does Hidden London walking tours of the lost parts of underground stations. We’ve done two of them: Baker Street (yesterday, as it happens), which involves a series of diversions from the public parts of the station through locked doors and into disused, long closed sections of the oldest of the underground stations; and Aldwych Station, a long closed but basically intact end-of-the-line station. It’s one of those things where, it it’s your kind of thing, you’ll likely love it.
I particularly liked the Aldwych tour; if you’re a film buff, you’ll probably recognize parts of the station. Here’s a brief introductory guide. Not all locations shown in the video are part of the tour.
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A Roman wall about 1800 years old among new buildings as you can see in https://www.youtube.com/shorts/3vQYNATZPno .
I agree! I’m one of those dorks who has a photo of both my feet, one on either side of the line. I also set my watch to the official time.
This is very true - you have some of the best restaurants in the world, and some right terrible ones if you get caught in a tourist trap. So plan ahead. Research your restaurants and, importantly for dinner, book ahead. You can’t just rock up at a great /popular restaurant, and expect to get a table. Planning is key.
Chelsea tickets can be had from resellers, such as this one.
You can’t mudlark without a licence, but you can take a tour which might achieve the same thing.
If you can, I would heartily recommend a tour of the Houses of Parliament - a truly unique building, and if you go when the House is not in session (Saturdays), you can stand at the dispatch box where the PM argues with the Opposition every week. You will need to book in advance as tickets are limited.
London obviously has a load of incredible museums, but if I had to pick one, it would be the V&A - a vast collection of art and design artefacts. They’ve recently opened a purpose build storehouse in East London (on the site of the Olympic village) which is terrific. If you fancy a swim, you can do so in the London Olympics Aquatics Centre which is practically next door.
I did this last year and it is indeed fantastic. You can just wander on your own, do the audio tour, or have a proper guided tour with a human being. We went with the audio tour which gave us plenty of info while going at our own pace.
But if you’re not into old buildings and the weird workings of the UK government, it may be less attractive to you.
I like that you go through into Westminster Hall first which is usually empty with nothing happening inside. But it’s got an insanely long history. In that room Henry VIII held his Coronation banquet. And it was 400 years old by that time. It’s also where they held the trials of King James I and Guy Fawkes.
The Guy Fawkes trial record is on display (under glass) at the Central Criminal Court (The Old Bailey). I don’t remember if it’s actually labeled as such – I only noticed it because a walking tour came by at the same time I was in the same corridor looking at something else.
Charles I!
Yep, the tour is super cool. You get to see all these storied places where all sorts of historical stuff happened. And find out that the House of Commons chamber is considerably smaller in person than it looks on TV.
Probably the coolest thing though was Westminster Hall, and the brass plaques in the stairs and dais part which commemorate where various things were announced, happened, etc… There’s one that I recall that said this was where William Wallace was tried in the 14th century, for example.
And the security guard will NOT let his eyes off you if you’re standing where the speaker is in that photo above. That podium/desk thing with the mace is chock full of House of Commons envelopes, letterhead, etc… and they get the same franking privileges that US congresspeople do.
(I was totally going to snatch an envelope and mail a letter to my brother in it, but that security guard stared at me non-stop while I stood there.)
Do’h!
I’m back from my trip as of yesterday evening. Some notes:
I stayed at the Gem Fitzrovia. I did a lot of debating about hotels - at one point I was going to commit to the Tower Hotel for the view of the bridge, but I’m glad I ended up at this one. I was a 5 minute walk from the Great Portland Street tube station, central enough to get me most places in central London with no transfers. Also very close to bus stops that ran me to Westminster and the like. Bunches of restaurants within very easy walk with lots of different cuisines - I regret not making a point of going to the Uyghur that I found on my next to last day!
The hotel was generally very comfortable - the mattress was very hard, but I adjusted within a couple of days. Breakfast is available in the hotel, which was nice to have for those days I wasn’t particularly motivated to go out and find coffee/breakfast early. Temp controls worked well for me - but I was also there in January, so wasn’t making use of any AC.
I used contactless payment for tube/bus and never had an issue except when I accidentally turned off NFC on my phone and couldn’t figure out what I’d done. Still don’t know how I did that.
Some highlights for me were the guided Parliament tour - as others have said, very much worth the time to do that. I also managed to get ceremony of the keys tickets for one evening by planning ahead and that was definitely neat to experience. Note that they release tickets 1 month ahead and they do book quickly. So if you were going to be there anytime in March, you’d need to try to get tickets at the start of February.
I went to the British Museum on a Saturday morning and…that was a LOT. I wish I’d done that on a weekday and maybe there’d have been slightly fewer people there. Still worth seeing, but did get overwhelming to me - I saw what I wanted to see though.
Saw a play - Ballet Shoes at the National Theater because I grew up a fan of the book and it was delightful.
Built in some time to wander and pop in and out of things, which is one of my favorite things to do in a city. One day - when my feet where exhausted after multiple 30k+ step days, I got the hop-on-hop-off uber boat ticket and spent some time on that, popping off wherever I felt like it and that helped me rest my feet. One of the places I got off was North Greenwich, where I took the IFS Cloud cable car round trip across the river. It was an excellent view of the skyline and just a neat take. I considered the London Eye, but the cable car fit that bill for me a bit more - it wasn’t the same view (I couldn’t see all the historical stuff from the cable car), but it was also a quarter of the price and I wasn’t packed in with a lot of strangers.
Enjoyed the trip. I think if I was deciding between a second trip to London and a second trip to Paris, I’d go for Paris - I just liked it better as a city, but I certainly had a good time and wouldn’t regret a second trip to do the things I couldn’t get to.