South bank on an evening, literally across the river from you ... lots of different things going on in front of the Royal Festival Hall and stretching towards the london eye ... never mind the views ... which I think are more impressive from the south bank over the river (Hayward gallery is also beside the RFH)

... everyone loves the natural history museum ...

If you go the Natural History museum and there's a big queue outside, go round to the side entrance on Exhibition rd. Its usually much smaller.

How old is your son, ruggs?

Meat mission in Hoxton is the best burgers around,

Pending the weather I would walk up to the river and then head up to Vauxhall, basically left, past parliament on the south side to Tamesis dock (if I spell it might), it is a boat, moored on the south side of the river. Nice for a beverage and not too tourist rammed at all. People already mentioned the graffiti tunnel by the station, Oxford street, good luck! For East I would either go to Liverpool St, or old street and walk to Colombia road, on a Sunday, flower market there, always busy, but nelly duff is there. If you walk round more that side there is a lot of urban art and you could go into jealous shoreditch ish. South bank has a food festival which is open at the weekends, street food. The biggest 3d cinema is also in Waterloo. Tate already mentioned. Not that I live here 😝. If you need more ideas let me know.

Forgot the Damien hirst gallery, which I yet to visit, but this side south of the river. Big stik piece by royal festival hall' under the bridge there. Have a pic somewhere, I am sure you will find it.