Where to begin? One of the most beautiful buildings in London is also home to one of the richest natural history collections in the world. This is also one of the few museums that pulls off the trick of being immediate and exciting enough for children while providing the kind of depth that keeps adults coming back time and again. Unbelievably, it's also free.
Cromwell Road, London SW7 5BD; Nearest tube: South Kensington; www.nhm.ac.uk/
Updates
Don't miss the gemstones gallery (in basement, I think. It's spectacular). You can also go into the Darwin Centre, (advance booking) and see actual specimens brought back from Darwin's travels. Better for the over 10's.
Is this your business?