Carcassone old town inside the walls is very pretty and if you are lucky they will have knights jousting which if you have kids is very exciting. We are spending 1 night there this Summer on our way to somewhere else. If you have a couple of days and a scientific interest then there is the solar furnace at Odiello up in the pyrenees. It has a visitors centre and they give quite interesting tours, they will even put the English version of the introduction film on if you ask them.

http://en.wikipedia.org/wiki/Solar_furnace