Day trip to London for dd's birthday (14yo). What to do?

[NoonarAgain]

dd is having a day out in London with 3 friends for her 14th birthday. Any ideas what we could do without breaking the bank?
She wants to go to Oxford Street to (window) shop and on the London Eye.
Any other suggestions?
I thought of the chocolate cafe at Harrods, but i'm sure MN have better food / treat related suggestions!

I should add that dd won't see going to museums/ galleries etc as a treat! we go to London a fair bit and have been to many of the museums.

Sky garden?

Camden market is good for teens
South bank as she will be on the eye anyway

thanks both. sky garden is fully booked. anything in particular on south bank?

What about the Emirates air cable line at the O2 arena. Then lunch at the O2 there are loads of restaurants there