Located midway between Milan and Venice, Lake Garda is large enough to straddle the provinces of Trentino, Verona, and Brescia and has enough thrills available to those who seek it for days at a time.
At the north end of the lake, you'll find some of the best windsurfing conditions in Italy, especially around Torbole and Riva del Garda. Malcesine, a city on the northeast shore, has a lovely little harbor, but is known especially for its proximity to Mount Baldo (over 2,000 meters tall). Take the cable car to the top, where you can rent mountain bikes and gear for the ride back down. The south end of the island is a bit more touristy, with a few well-kept, man-made beaches and a popular theme park called Gardaland.
For more extensive information on the towns you'll see on the lake, as well as other activities (like the obvious swimming and sailing) that will keep your itinerary full and potential for great memories high, check out the included link.
Note:Nearby Arco (just north of the lake) is quite popular with rock climbers for the dozens of limestone crags available.