We used to visit North Wales a lot, we had friends who lived near the Lleyn peninsular. I have to be honest and say that our idea of a good time is a long coastal walk and a visit to a local museum which - I'm guessing - may not be your thing
There are some fabulous beaches up there.
In South Wales, I've only ever been to the Gower Peninsular, which is very beautiful but, once again, wrinkly fodder, rather than a place for teenagers. There are, once again, lots of beautiful beaches and both Swansea and Cardiff have most of the things you'd expect in a big city, like leisure centres, music venues, cinemas and theatres (which will almost certainly be doing light-hearted summer stuff during the holidays - Google is your friend).
A word of warning - in Wales, it can rain - A LOT - and then rain some more. Don't let that put you off, but pack waterproofs as well as beach wear. Hope you have a great time
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