What is a fun and cheap activity you can do in London when it's summer?

In the summer it is nice to go to St James park, where you can feed squirrels and birds such as geese. If you first go to the Tesco's, you can buy some peanuts for under £2. At the park there will be several squirrels, little green birds, geese and swans. They can eat the peanuts out of your hand. For the little green birds you may have to peel the peanuts first, but it's worth it when a small green birds sits on your hand.