One and a half days in London: where to visit?

[Mondaynightblues]

Have booked to go to London in half-term week, arriving into London late morning and leaving late evening the following day. I'll be with DH and DS aged nearly 16. Booked a while ago and has rather crept up on me, and now I'm thinking I'm not really sure what to do when we are there. DS would like to visit an art gallery, so I'm thinking that will take about half a day, but I don't think any of us are so into art that we want to spend the whole time visiting galleries. If weather is dry, would be nice to spend some time just going for a wander and taking in our surroundings. (DS is very into taking loads of photos.) Last year we walked along the South Bank from Tower Bridge and also walked from the Houses of Parliament to Buckingham Palace. Anyone got any suggestions as to where else would be good for a walk or any other suggestions? I'm conscious that with it being half-term, some visitor attractions will be heaving, and I don't want to spend hours simply queuing to get in somewhere. NB We already have plans for the evening (meeting friends), but I'm a bit undecided what to do during the day. TIA

You could walk around Greenwich - the park is lovely, there's the naval academy, Queen's house, the Royal Observatory, The National Maritime Museum (free and way more interesting than it sounds) The Cutty Sark ship and loads of cafes and market stalls. Then you can get the Clipper boat back to the centre of town for about a fiver.
Or walk around Hyde Park and Kensington Gardens, look in at the twin Serpentine Galleries for good, free modern art exhibitions and then stop at Kensington Palace for tea in the cafe there.
Or go on the street art walking tour (also about a fiver) round Spitalfields market. That's brilliant.

Thank you for the suggestions. I had wondered whether Greenwich might be a good choice. Had not heard of the Spitalfields market walking tour. Will look into that.

As a rainy day option - I love the Museum of London. It's in the City and it much less popular than the Natural History etc so probably won't be as packed. There is also a museum at the Bank of England which I've never managed because it's only open on weekdays. But I've been told it's really interesting.

YY to the Museum of London - the Fire! Fire! exhibit is really interesting.

Also good is the Treasures Gallery at the British Library - not really suitable for little children, so shouldn't be too busy in half term (although there will be children around).

If DS likes photos, you can go on architecture tours of the Barbican. I haven't ever done it, but I imagine there will be lots of opportunities for good, arty photos.

Sir John Soanes Museum at Lincoln's Inn Fields, the Geffrye Museum and the Foundling Museum are all really interesting, smaller-scale museums. I sobbed my way around the Foundling Museum.