We will be there the 2nd full week of April. So we expect rain, but that's okay.

You could always try mid-Wales as a compromise. Little villages, great views, farmland and quiet countryside. The Brecon Beacons National Park is good is you fancy getting away from buildings and into nature.
http://www.breconbeacons.org/

If you're settled on South Wales I recommend checking out Cardiff Bay, especially if it's a nice day. There's a few things to do there and it's easy to get to if you're already in Cardiff.

I totally agree with the castle recommendations, Castell Coch is quite a traditional fairy-tale looking one. You could also check out Caerleon near Newport for Roman ruins if history is your thing.

As for accommodation I suggest you check out http://www.southwaleshotels.co.uk/ as there's a whole lot of options listed on there. You can put in a few specifics too, price, star rating, etc.

Hope I was some help :)

South and mid-Wales are both great locations. Castell Coch is great as is Cardiff Castle.

If you need more info you can use this Welsh Holiday government brochure site that's very easy to use, you don't have to register before ordering and the brochures arrive in a couple of days.

http://www.wales-holiday-brochures.co.uk

The brochures helped me...

you could certainly see a lot of south and mid-wales in 3 days, but going north will require more hard slog and less fun.

i would aim for the area between the Brecon Beacons and Cardiff. hard to go wrong, really.