Caernarfon is nice, and there's a castle there for a look around, it's not too much of a drive, and there's a motor museum on Anglesea if that's your thing. Black Rock sands nr Porthmadog is good for kite flying. If you can, try to get to get to ty Coch inn, it was voted one of the best beach bars in the world a couple of years ago.
Oh and lastly, if you happen to head home through Welshpool you should stop at a butchers there called Rikki Lloyd. They do the very best handmade pies I have ever tasted and always stock up for my freezer on the way home