Brilliant! Somerset House, Strand, London (sorry, don't remember the house number). The courtyard has a marvelous fountain display, free to enter, seating and tables around it, no traffic. We just stripped the children down to briefs or nappy and let them run around in the fountains. It didn't take long before dh joined them! Remember to take towels and a change of clothes as they'll want to go back in again as soon as they've dried off. You could also try remembering to take swimsuits - we forgot! Toilets and food available inside, though you have to dry off first, it is a stately museum and part of the Inland Revenue after all.
Shame the weather's just changed.