Oh, agree with St Fagans and Big Pit is also good (ok, it's a...errr...big pit but an interesting one!).
http://www.museumwales.ac.uk/en/bigpit/
Oakwood is the themepark but Alton Towers in aint!
Oh and Barry Island is a must!!! I haven't been back since I was a child so apologies if it's now a dump (whilst on that note, if in North Wales, give Rhyl a miss!).
Worth checking what's on at Cardiff Castle, they sometimes have outdoor events on in the grounds and it's lovely there!