Santa Theresa is a nice town with some good restaurants, but what you come here for is the water. Breathtaking scenery.
Bring your bathing suit, once you see the beach you'll want to go in. It's really some of the prettiest water I've ever seen.
Take a day trip to Corsica. The boat leaves from the port of St. Theresa. It's about a 45 minute boat ride for about 35 euros RT. The boat ride is very calm and nice, no wind and calm seas. It lands you into the lovely and interesting port town of Bonafacio. You can bring a rental car over and drive around the island or you can just hang out in Bonafacio. There is plentyto do, nice shops, restaurants, an old fort and cruises that take you to snorkeling destinaitons. You could spend a day in Bonafacio and not be bored. it's a very pretty, charming port town.