Free museums: Natural History Museum, Science Museum, and V&A — all free, all amazing for kids.
London Zoo: Regent's Park, 750+ species. Book online for discounts (£27 adults, £17 children).
SEA LIFE London Aquarium: South Bank, 600+ species. Combined tickets available with London Eye.
Warner Bros. Studio Tour (Harry Potter): Must-book in advance. Located in Watford, 20 min by train from Euston. £53 adults.
KidZania: Indoor city for kids to role-play jobs. Westfield London, Shepherd's Bush.