Oh! I can help!
Within Cardiff centre there's a few nice options. If they're up for curry I can strongly recommend Vegetarian Food Studio in Grangetown, just over the bridge and about ten minutes walk from the centre. They're pretty cheap and the food is amazing, plus a very friendly ambience. If you're after curry with meat then Eurasian is pretty good on Cowbridge Road East, but not as good as VFS.
I also recommend Casanova, practically next to the City Arms which is good for a pre-meal beer.
http://www.casanovacardiff.co.uk/They do Italian food but it's more of the provincial peasant type, less of the pasta and more of the rich meat dishes. Great place and once when our food was a little slow in coming (about twenty minutes or so late) they gave us a free nice bottle of wine and threw in some starters.
For sushi try Tenkaichi on City Road -
http://www.tenkaichi.co.uk/ Pretty good stuff and a league ahead of the other sushi joints in town.
The Meating Place on St. Mary's St. has been getting good reviews from friends and others. I've not tried them yet but it's a good bet if they're after steak and pie type goodness.
http://themeatingplace.co.uk/locationIf they're there in the daytime Cafe Minuet in Royal Arcade is great. It's not at all a romantic setting and can be crowded in its tight confines, and it closes at 4:30, but the food is excellent. Especially the soft cheese and mushroom / spinach pancakes in delicious sauce.
Hope that helps!