There is also surfing, and usually beach parties too. And it's cheap. My first holiday without parents was a BTCV conservation holiday. Paul Chapman's daughter and her friends could try youth hostelling. I believe that the YHA will accept bookings from unaccompanied teenagers.
Some hostels run special interest holidays.
They are good value as far as price goes, offer various combinations of self-catering or meals. You also have the chance to meet people from other countries. The city is walkable so the fact that we couldn't drive wasn't a problem either.