Bath's top visitor attraction is the famous Roman Baths Museum. You'll need a couple of hours here to see it all and get real value for money.
Just a warning -- there is a mostly good, but interminable museum between the upper and lower views of the baths. If you are short of time, don't get hung up there!
A Reconstruction of a 2nd century C.E. temple dedicated to the god Sul - this place was then called "Aqua Sulis". Perhaps more fragments will eventually be found!
The Bath complex is actually on two levels - the contemporary street level, and underneath, the original baths themselves, which were discovered under some twenty feet of accumulated dirt and debris.
The roman baths were restorated during the Victorian period and are still fed by one of Bath's three hot springs. In the museum there are examples of Roman sculptures, mosaics and other arts.
You have to go see the actual Baths. That's what all the fuss is about. Try to go in a non-peak tourist time though. The recorded tours are worth doing.
When in Bath you must visit the Roman Baths! I believe you can get in free if you can proof you live in Bath!
The self-guided tour is really interesting and takes a good couple of hours to look round the whole museum. If you can visit on a sunny day, it's lovely to sit by the water for a while.