Going to a westend show in London is a great day out, I recently went to see 'Dirty Dancing' and thought it was fantastic. 'We will rock you' is also a brilliant show.
If you want to go out too seeing as you are now legal to drink then you could stay over in London and go out after.
Or for my sister's 21st we rented out 3 small cottages for the weekend. Was really good fun! Can't remember what the place was called, but it was in Surrey. Worth having a look at - xx