Recommended places to stay in Cardiff:
The Riverhouse Backpackers is highly recommended, close to the centre, right in front of the Millennium Stadium, a great garden and kitchen and a very pleasant atmosphere. Abi and Charlie will make your stay in Cardiff unforgettable.
Right next to it is NosDa, slightly different and with a great pub and terrasse area. So if you prefer something more lively and for bigger groups, this is a great choice.