If you're going to Wales, I highly recommend Bangor and/or Anglesey. It's a bit of a pain to get there (although there is a direct train from London), but it's well worth it. The train journey up the coast is really beautiful, and you're close to cool stuff like Conwy Castle or Church Island on Menai Bridge. (That may be too touristy, I don't know...)

You didn't mention England at all, but have you considered stuff like the Uffington White Horse or indeed any of the other chalk hill figures which are scattered around in various places? They're pretty impressive (although often somewhat difficult to get to).

Are you going to have a car, or are you going to be only using public transport?