You may visit the two old trees. It is said that the older has an age of about 3000 years. They look old, thats for sure. You have to pay a few Euros for visiting them. If you visit from November to March you will most likely pay nothing. The cache can be accessed without spending money. And maybe you want to take a coffee at the bar. There is a beautiful sight on the Lago di Liscia.
When you drive by car take care! You might think that there are small stones on the road - but these might also be tortoises! I have been there several times and seen a few of them sunbathing.
Note: I want to note that you cannot cross the dam (diga di Liscia) via SP137. That means if you come from the east (Sant'Antonio di Gallura) you have to go around the lake (clockwise from Sant'Antonio)