Us Cardiff girls seem to be hijacking this thread for ourselves!
My fave eatery is a few miles outside Cardiff in Pontypridd. It's called The Bunch of Grapes. It's a spit-and-sawdust locals pub at the front, but it turns into this amazingly chic French restuarnat at the back, kind of like a tardis.
If you like quirky places there's a family-run Caribbean restaurant down the bay, on Bute St. It's tiny, like the menu, and it's like someone's front room, but the food is good, and authentic. Also, for lunch check out Cafe Minuet in the city, it's in one of the arcades. It's run by this Italian opera buff. It's tiny so get there early. Massive rustic portions.
As for days out, agree with everyone else. Just wanted to add Techniquest down the bay for kids, and if you like arty places where you can eat, watch films, and drink Belgian beers, check out Chapter Arts Centre...
Agree that Cardiff looks like a dog at the moment!