London is so vast I am forever coming across new hidden gems. Walking around the streets and getting lost is the best way to find London's best kept secrets.
The Royal Park offers great views of London and the city; As well as housing the Royal Observatory which includes a museum of astronomical and navigational tools, part of the National Maritime Museum.
There are not many places in London like it. On a Saturday during market hours it heaves with stalls and visitors for a good mile or two. Great place to pick up some bric-a-brac, genuine oddities and a great bite to eat, or just enjoy the hustle and bustle vibe of the place.