Make hotel reservations if you plan to visit in June, July or August. Also, be aware that hotel and other rates go up during this time. So you may wish to consider off-season travel for the savings. You will probably be flying into Milan, Verona or Brescia airport, then rent a car or take the train or bus to the Lake Garda region.
Once at the lake shore, ditch the car for some serious ferryboat rides. Ferries link all the resort towns on the lake. Speedboats, catamarans and hydrofoils slice through the water for a faster option. For some romance, or just fun with the kiddies, try the paddle boats (rowboats) or pedal boats. If you wish, ferries that transport cars are also available. Boat service decreases from October through April.
Stroll and eat. Stroll the boardwalks for some serious people-watching and shopping. Sit at outdoor cafes and "mange" to your heart's content on fish specialties, pizza, pasta or gelato. Or simply nurse a cappuccino. If you are lucky, catch an open air market (sometimes a combined food and flea market) to get of the taste of the truly local.
Bask in the sun at one of the hundreds of sandy lakeside beaches. Whether swimming or sunbathing, you can soak up the Italian sunshine any way you please.
Try a sport: windsurfing (even kite surfing) is huge here, and you can take a class at one of the many windsurfing schools; but hiking, biking, climbing, canoing and kayaking opportunities also abound. For some deep post-sport relaxation, go for a thermal or mud bath at a hot spring in Sirmione.
Explore the ancient ruins. You will be surrounded by castles, cathedrals and villas that are hundreds if not thousands of years old. They will be set off by lemon groves, olive orchards and cobblestones. Climb all over these archaeological wonders. Also, visit the numerous museums to appreciate the historic details.