Free easily accessed things in London

So with the weather getting unseasonably hot and fine so much so early in the year I want to fight the winter blues by getting out and enjoying some of what the city on my doorstep has to offer. Come on hive mind - what have you got?

If you want context for me personally, I'm on benefits and have a disability that means I can't walk for more than a fairly short distance so everything needs to be close to a tube stop and preferably free (the cost of TfL and a sandwich will max my budget in itself).

What do you suggest for this? I'm into art galleries and museums (if they arent too hard on the feet) and both high and very low culture (tho I find middle of the road stuff boring and certainly can't afford to be ripped off by paying through the nose for some "experience" that isn't really all that; Im looking at YOU 3D "immersions"!). Visiting parks and stuff is fine but not worth going into the city for IMO. But what you are into is just as helpful in answering the question cos widening horizons is also really good.

TYVMIA