Are you a Doctor Who / Torchwood fan? Cardiff is full of filming locations, like the Roald Dahl Plass at the bay, the water tower, the Millennium Center, and the Ianto Shrine. There is also the Official Doctor Who exhibition with costumes and props and a show at the beginning in which you can participate by flying the TARDIS. It sadly closes after this Summer though.
Half an hour away by bus is the Museum of Welsh Life in St. Fagans. I love it. A huge park where they have transported old houses to from other locations to and rebuilt them and there is old furniture and you can look how people used to live. A whole village with post office and school and farm and shops (it's also a Doctor Who filming location).
I guess in Summer you can also do boat trips in the bay but I always was there in harsher weather.
There is an American styled diner in the bay which - you guessed it - also was a Doctor Who filming location. There are pictures and autographs on the walls. Actually we were there before they filmed and then got a surprise when seeing it on screen!
