There is a travel lodge down the bay, easy to walk to and from.
I haven't been to he dr who bit, but there is a boat trip from the bay to town down the Taff. It's really good fun and drops you off by the castle. Or stay in Cardiff town and get the boat or bay bus( which is very regular) down there.
Have a look what's on at the millennium centre, as you are in the bay.
There's also a cinema down there too, as is a nando's and pizza express.
We are taking DD to Cardiff in early April, we are planning on booking the Doctor Who bus tour and the Doctor Who experience, and going to Techniquest. We'll apparently also be spending a ridiculous amount of time on Mermaid Quay going "ooh ahh" at the Torchwood paving stone.
Any recommendations for a decent but fairly cheap hotel/B&B? Plus can anyone say if the bus tour and/or experience are worth doing?