North Wales has the most dramatic landscapes with Snowdonia but anywhere in Wales is a good choice. They have recently opened a coastal path around the whole of Anglesey which also has some great beaches. Mid Wales is the least populated and there are some good spots around the Llandridnod Wells areas. Pembrokeshire is also famous for it's coastal footpath and we had a brilliant trip there in April.Even the more populated south has some great spots too, the area around Swansea, the Gower, Wye Valley, Brecon Beacons... cardiff is a great city especially if the weather is poor.
I am biased as I live in North Wales, so that is where I would recommend !
Any of the CC sites in Wales would do nicely.
Come to think of it, I think we'll have a trip in Wales again next year !