The spas, especially the outdoor ones. There is no better cure for the night before. Bath is such a beautiful city that loads of the clubs have to be underground to avoid ruining it. You can wander from one end to the other in under half an hour. Awesome.
Watch out for dodgy air-con in some of the clubs. BarChick had to dodge the drips. This in hen-do central, if you're on one, make sure to keep with the right group! The city is on a hill - be prepared to walk up it!