Cardiff has developed two tourist areas - the main centre or the Bay. City centre has usual chain hotels from Travelodge to Hilton, shops etc. The bay has a number of hotels and loads of restaurants and bars. Parking will probably be easier in the Bay and it's only a five minute bus, rail or taxi journey between the two.
Bar Cwtch which is in the cellar of a boutique hotel is one of our favourites:
http://www.jolyons.co.uk/index.htm
Mary Street/Westgate Street near the stadium has loads of bars.
At railway station end of Mary Street is Mill Lane with numerous restaurants - Le Brasserie is an institution in this area and is a good place for a steak.
If they want the true Cardiff eating experience I suggest a pub crawl down Mary Street from the Castle to the railway station, then chips and curry sauce from Tony's on Caroline Street - for the full experience this must be done after midnight!
I remember going there once and someone had parked on Caroline Street (I don't think you can now as its gone all upmarket!), but they had left a window slightly open. Being people who want to look after their environment and not litter the street the car became a convenient litter bin and was slowly filling up with chip papers!