what should we do for one day/nght in London?

For the amount of time you have i would take the tube to Piccadilly Circus. There is a lot to see and places to eat. You can walk from there to Hyde Park. you can go to Fornam and mason for Tea. Or go to Notting hill. it's really lovely.

It depends where you're staying. If you're near enough, hop on the tube or a bus and make your way into the West End - where there is just about everything you could desire. Alternatively, find some decent pubs near where you're staying. Don't stay in your hotel - that's boring.

London is a very safe city, so don't worry about wandering around the centre. Pubs are probably the best and least expensive entertainment centres in London (there are around 5,500 of them) but if you don't care for pubs then get hold of a copy of Time Out and plan from that.