Snowdonia is great, spent a couple of weeks there last summer in the van. The roads absolutely rock, I even managed to have some fun with the van round the twisties once I blocked out the fact that there's nearly always sheep round the next corner!!!
If you go anywhere near Llandudno go a drive round the land mass known as 'the great orm'
It's got a one way toll road running round the coastline with loads of lovely bends/hills, you can blast round as many times as you like once you have paid the toll (about £2).
