I love Saturnia, it's very scenic! The water coming from the official spa, gets out and becomes the river Albinia. At the very first part it makes a nice waterfall, then continues towards the sea. The part around the waterfall is really cool: the water is hot, and it's not too smelly! And of course it is free.... I went there a couple of times sleeping next to the river with my tent.
The easiest way to get there (coming from Rome) is from the highwah 1 (via Aurelia) to Montalto di Castro, then to Manciano, and finally to Saturnia. It takes 2 hours or a bit more.
If you like thermal baths, there are several places also around Viterbo. It is less than a hour from Rome, and it could be a cool way to have a GTG.... I may take you there if you like!