Can you think of places i can take my parents, that dont cost money, or a lot?

My parents are coming this week, for a holiday. i was trying to think of a couple places to go in london, that dont cost a lot of money. i know like, walking around london, science museum. can you think of anywhere else. it will be really appreciated!! thanks in advance! x
Answer
The south bank and covent garden , great entertainment all for free , apart from putting a few quid in the hat !