Nature in Tuscany is wonderful. It covers almost every aspect of Italian geography, from islands to mountains. But its gentle hills are what everybody recognizes as the main particularity of its landscape.
From the geological point of view, waterfalls are very rare in Tuscany. However there’s one waterfall worth to mention: Acquacheta. It is in northern Tuscany in the wonderful national park called “Foreste Casentinesi” (here you can find some useful information).
Tuscany is also famous for its thermal springs and.......art, obvious!
Then, another option would be to head down south and visit the famous Saturnia thermal springs where you can find the “Cascate del Mulino” and “Cascate del Gorello”. However, there’re not big waterfalls, being Saturnia basically a spa, but the place is a true gem.
if you're not looking for "big" waterfalls the places I suggest are both perfect
Perhaps they may not look like the typical Tuscan places, as everybody thinks Tuscany just means vineyards.
In fact Acquacheta is part of a National Park and is in a region north of Tuscany famous for its mountains and wonderful woods, while Saturnia is south of Tuscany where landscape is hilly and is famous mainly for its thermal springs.
But I can assure you they're amazing. If you're looking for gentle hills and "country mood"....well, lets' forget about waterfalls and let's head to Chianti or Val d'Orcia. Both are maybe what Tuscany evokes most frequently in people: here you can read a useful guide to Chianti.
As for the second region, here you can see some pictures of Val d'Orcia and what its landscape looks like.