For participants who are happy to travel slightly outside Cardiff, we recommend visiting Caerphilly Castle, a very impressive ruin in the town of Caerphilly just north of Cardiff.
To get to Caerphilly, take a train to Cardiff Queen Streat Station and change there to a train to Caerphilly Station. That castle is then a short walk form the station.
If you would rather stay in Cardiff, you can explore Bute Park, which is behind Cardiff Castle (also worth a visit), or you could visit the Cardiff National Museum.