When I was a nanny I used to take the children to Morden Hall garden centre to look at the fish - as it was indoors and free! Also why not go and spend an hour wandering round Pets at Home - you can see rabbits, snakes, chin chilas, fish - the one on Plugh lane in Tooting/Wimbledon is not that big but enough to occupy for 40mins? Just don't come home with a pet