Best rainy day activities in London for a family?

Science Museum, things to do, see and be amazed at, and it's free!

I dont really see the appeal of going out on a family day whilst it's raining.... I would stay home and watch movies all day. If you have children, films like the goonies are great for a rainy day but if you really want to go out I think the London Aquarium would be great fun.

Museums are always a good bet. There's three of them together at South Kensington - the Science Museum (with the Garden and Launch Pad areas downstairs for kids), the Natural History Museum (with cool interactive dinosaur exhibition) and the V&A Museum with vintage clothes etc. There's the Namco arcade centre on the South Bank, lots of family friendly shows in the West End that have matinee showings etc. Not sure if the aquarium is open yet, it was being refurbished but it's on the South Bank and huge. For lunch there is the rainforest cafe at Piccadilly with moving animals etc. Most kids like the Tate modern gallery because the stuff is so random / huge and they can just wander around looking at stuff. There is always the cinema - there's a good on at Whitelys shopping centre near Bayswater tube, the odeon at Marble Arch and three cinemas at Leicester Square. The National Gallery is quite good for a wander around and has Van Gogh's Sunflowers. The British Museum has a fab Egyptian room which has the Rosetta Stone which is well worth a visit. If the kids are not too young then the London Dungeons at London Bridge or the London Bridge Experience might be fun. There is always the overpriced tourist trap that is Madame Tussauds at Baker Street but I don't think it's worth it unless you can find some discount vouchers to use.

the tate moderne museum! or maybe london aquarium

Apart from the obvious places such as the Science museum or the Natural history Museum there is Funland in the Tracadero at Piccadilly circus. Dodgems, Bowling, high tec Video games, Virtual reality simulators and music as well as cafes bars and restaurants. It's great fun.