Suggestions for an European trip?

I am going to London with my sister next year. We were planning to just stay in London, but it seems better to visit other cities while there. The plan right now is to arrive in Amsterdam, stay 1 night at a friend’s house, then stay in London about 5 days, move to Paris about 2 nights, and then we have 6 free days. We are thinking of Barcelona or Frankfurt, any suggestions?