Cardiff Castle is also lovely, and if you like castles the Caerphilly Castle is also a short train trip away. Mermaid Quay/Roald Dahl Plass/the Millennium Centre is all a very lovely area too (I like to just hang out there when I go!). There's also a nice walk around the bay to Cardiff Barrage and from there to Penarth, Penarth Pier is very pretty. If you're into that sort of thing, there's also a museum/gallery. Oh! And there's a permanent Doctor Who exhibition with props and costumes and things.

My Wales flickr tag is here (though that includes some roadtripping I did around South Wales).

Also, you can eat Welshcakes, laugh at the sheep poo paper and attempt to read the bilingual signs. IT'S A VERITABLE PILGRIMAGE if you're a Whoniverse fan.

ETA: that said, I've spent some time in London too and can help with touristy things there too :) (If zortified didn't mention it, I am an Aussie being an expat in the UK at the moment, I've only been here since January!)

(edited again because I can't spell)