The Horniman Museum is one of the best free days out in London, and almost nobody from north of the river goes. Frederick John Horniman was a tea trader who collected everything he found interesting — musical instruments, ethnographic objects, taxidermy — and gave it all to the public in 1901 along with his house and garden.
The building is Arts and Crafts by Harrison Townsend; the collection inside ranges from a giant stuffed walrus to a library of global musical instruments. The gardens are terraced into the hillside with a view north over south London that on a clear day reaches to the Shard. Come on a weekday when the school groups aren't in.