Kew Gardens (tree-top walk in particular; take picnic, obviously weather permitting) Docklands Museum (brilliant fun with the added fun of travelling there on the Docklands Light Railway Greenwich Maritime Museum (go there by boat if it's not too cold; we go from London Bridge) Houses of Parliament (not sure if open in October) British Museum (ask about children's free packs - they follow a trail round an exhibit, which makes them really look at what they're seeing) London Aquarium (but get there as it opens and see above comment on 2 for 1) - it's even better than before. Tate Modern (ask for children's pack and even non-arty children will enjoy describing what they see)
I have been looking everywhere for inexpensive ideas to keep my little ones amused during the October half term! I've previously taken them to loads of different tourist attractions but wanted to find some stuff they could do that wouldn't cost a fortune!
I came across the Betty Crocker Family Fun Finder (www.bettycrocker.co.uk/great_idea.aspx) and found it to be a huge help! There's some really simple and great ideas on there for all different age ranges!