The Baths of Bath. The History dates back ~2800 years. The latest buildings however are only ~220 years. Still, enough to make it a UNESCO world heritage site.
It is Britains only water hot enough to be called a hot spring. The volume is enough to fill several large pools.
How much more fun would it be to actually go in the water though ;) I wonder how many people have "accidentally" fallen in over the years.
It was interesting to listen to the history as we walked around, quite a bit of the ruined roman construction still exists.
Later on we met up with Roy and Midge, we wondered around town dodging the rain storms and seeing a couple more sights.