I would suggest heading to Gloucester and driving down through the new forest, then pick the motorway back up (M4). Be careful of speed patrols, they are much more frequent in Wales than England.
It will cost more in fuel than paying the bridge charge to go via the forest, but the new forest is beautiful.
Once in Cardiff, there isn't alot to do AFAIK.
The castle in the city center is supposed to be worth a look, and the city is good for shopping, but it is a student based city mainly.
There is loads to do on the outskirts, loads of protected land, walks and ancient artifacts to visit.
As you leave Wales, you will drive over the 'New bridge', as you 'Land' back into England, there is a village at the foot of the bridge on your RHS, be sure to give me wave, that's where i'm at....