Cardiff's food scene has improved dramatically in the past decade and is now well worth seeking out beyond the obvious tourist areas. The city centre and the inner suburb of Pontcanna have the densest concentration of good restaurants.
Asador 44 on Quay Street is the place for Spanish-inflected cooking — the suckling pig and dry-aged steaks are the main event, with a serious wine list to match. Hang Fire Southern Kitchen in Barry (a short train ride away) does the best American barbecue in Wales; expect to queue on weekends. For something lighter, Milkwood in Pontcanna serves excellent modern British food in an unpretentious room that fills with Cardiff professionals most evenings.
The city also has a strong café culture — Uncommon Ground near the university and Hard Lines in Roath are both good for coffee and something to eat during the day.