Went to Bristol for mine and thought it was a great town. We went to Cheltenham racing on the Saturday - only 30 mins by train.
Newcastle was great when I went a few years ago. Great size of a place, locals were friendly, bars were good and varied. Although I was there again in August and I've a feeling I've got too old for that town (I'm an old 31).
I wasn't overly impressed with Liverpool, but I'm awate this makes me the exception. Everyone I know raves about it. Personally I don't like getting accosted by tramps selling knocked-off merchandise every time I sit down for a pint.
Carrick On Shannon was excellent. Small towns suit me better.