plenty to see and do in Cardiff, what sort of stuff are you after?
you can bike ride down the taff trail from Castle Coch, through the centre of Cardiff via Sophia Gardens, stop, have a look around the castle, shop in the victorian arcades then get a £4 river taxi down to the Bay where you can pick from dozens of restaurants and bars, see a show at the Millenium, do the Dr Who Exhibition or just eat icecream crossing the barrage
if it's sporty stuff you're after you can catch some cricket at Sophia Gardens or get wet doing the Cardiff International Whitewater rafting centre
some good clubs in Cardiff and a decent representation when it comes to live music, from small intimate gigs at the Globe or Clwb Ifor Bach or bigger stuff and plays and whatnot at New Theatre, St Davids Hall, The Sherman, Chapter Arts and the Millenium - don't be put off by Clwb being described as a welsh club, perfectly friendly and english speaking
a short hop from Cardiff is Penarth with a trad seafront and a pier and Cosmeston Mediaeval Village and walk around some lakes and ducks n stuff
or you could go to Rhyl and buy dirty smack for £20