In Cardiff you will find the hop on - hop off bus. Ideal for sightseeing Cardiff. For 2009 there were 10 stops:
Cardiff Castle (Castell Caerdydd)
National museum of Wales (Amgueddfa Cymru)
Holland House Hotel (Fitzalan Place)
Cardiff Bay (Bae Caerdydd)
Craft in the bay
St Mary's Street
Millennium Stadium (Stadiwm y Mileniwm)
Price in 2009 is:
Adult GBP 9.00
Child (5-15yrs) GBP 4.00
Senior (60+) GBP 7.00
Student (requires valid Student ID) GBP 7.00
Family (2 Adult + up to 3 Children) GBP 22.00
The ticket is valid for 24 hours.
If you have no idea about the Cardiff, why not take a city tour bus look around the whole city first. They also offer you the professionl tour guide tell you the many history about the City and every building, gardens as well. I suggest you take the tour in the moring, you can get off the bus at the Cardiff Bay and have lunch and enjoy the new developing business area, then catch the last bus come back city centre.
The tour bus station just outside of Cardiff Castle and you can have discount with student ID.