What to see in Wales

Not very well. Wales, of most tourist interest (the castles), is along the coast. Picture a 'c' starting at Chepstow and ending headed toward Chester. This misses Anglesey (which is proably okay with your time constraints) and also Snowdonia (which you really ought to dip into). Driving time for the 'c' described above is less than eight hours. From Bath to the Severn River bridge is a half-hour. My picks for great castles (there's a couple hundred total), in order and with the very best in CAPS would be: CHEPSTOW, CAERPHILLY, Kidwelly, Pembroke, HARLECH, CAERNARFON, Conwy. Each would take almost a couple hours, Caernarfon a bit more than three if you spend any time at all in the regimental musem. Harlech is the best of the best, little Chepstow a close second. You can park adjacent to all of them. A half-day is enough for Cardiff (skip that castle), but even the city is not essential. If you arrive at Caernarfon late, you can see the castle the next morning, swing down into the Snowdonia national park, mess around, and still be in Conway in time see that castle and have supper. If you're thinking about the railway, don't make reservations since the whole route may be clouded in. I don't think you'd be at of not getting a seat since the heavy tourist season would not have started. With the same logic, I wouldn't even worry about reservations - - there will be plenty of vacancies in the bed and breakfast places. You're probably going to have to eliminate some of my list of castles and then plan your overnight stops.

The northern coast of Wales is stunningly beautiful. There is a village near Snowdonia Nat'l Park called Betws-y-coed that has lodging, restaurants, and shops. Many people pronounce it Betsy Co-ed, but the Welsh really appreciate it when visitors pronounce it: Bettoose-uh-coyd There is a tea house that is a must in the town of Llanrwst. It's called Tu Hwnt I'r Bont. I have no idea how to pronounce it, but it means Beyond the Bridge. It's on the banks of the River Conwy, and was built during the Tudor era. Everything is homemade, and they have the best scones I've ever tasted. You cross a one lane stone bridge to get to it. There are no toilets, so customers have to use those located in the public park across the street.
The park has a stone circle with an altar stone in the middle. www.betws-y-coed.co.uk There is a scenic train ride up to the top of Mount Snowdon that is worth a look, I was told. It was raining the last time we visited, so we didn't go.

In an earlier post Dave mentioned he was staying in Beddgelert. I had to agree that it is a lovely place to visit and to use as a base. Ed is right, there is no location from which you can easily see most of the country. We have also stayed Criccieth and it's easy to get to Port Merion from there too.

If you are near Chester Wales it is a lovely little border town

Chester is a brilliant idea, with the overhanging walkways and city wall. But, it is in England. Not by a lot, but it is England.

I recommend Betwys-y-Coed too. Lovely little place! I am also very very very fond of St. David's, about 3 hours drive to the south, out at the tip of Pembrokeshire. The cathedral is stunning. Driving along the coast would be my ideal way of seeing Wales. You cannot beat the scenery. Agree with Ed about Harlech and Caerphilly. Ed gives good advice, IMHO. And please do not go to Wales without at least poking your toe into Snowdonia! If you want a bit of local history while you are there, check out the slate mine tours at Llechwedd Slate Caverns of Blaenau Ffestiniog.