If you like some nice cool beer, portuguese sweets (the once with yellow custard inside) tasty and affordable sandwiches, good strong coffee and football, at all hours. Max Snack Bar is worth checking out.
London is so vast I end up spending hours travelling on trains. They are by far the best way to travel around on and are always full of weird and wonderful characters.
Everyone should visit this absurd restaurant once in their lifetime. Its over-priced, mediocre food but the decor and ambiance is like nothing else in theatrics and campery. You are met on arrival by a white velour 'napkin' artfully screwed up on your diner plate. Absorbent, not? Fabulously awful, yes.
On a hot day there is nothing better to do in London then go for a swim in the Heath ponds. You get completely transported you out of the city as Hampstead feels like being in the countryside except you're in central London?!