Largest and most rugged of the Italian lakes, Lago Garda is less than an hour's drive from Verona. Surrounded by peaks that mark the beginning of the Dolomites, the 48x16 km lake was chosen out of all of Northern Italy by Mussolini as the seat of his 1943-45 puppet Republic of Salò (this was after the Germans sprung Il Duce from his prison cell in an Abruzzo ski lodge).
You can cruise around the lake on a public steamer in about three hours (this
is the way to see all the best villas and gardens) or drive around in something
like four (this is the way to see some amazing tunnels; the cliffs on the west
shore are nearly vertical so about 50 tunnels carry the road through).
Everybody's favorite part of Lake Garda is the car-free peninsula of Sirmione, on the south shore near the Venezia-Milano autostrada. You park your car about 1 km from the end of the peninsula and walk over a stone bridge smack into an interesting castle built by the Scaligeri of Verona. Medieval "Downtown" Sirmione is here as well, filled with nice shops. I visited on New Year's Day and the place was reasonably packed with Italian women in fur coats and Austrians in technicolor nylon outfits. Of course, everything was closed so folks just milled around. The tranquil Cypress-studded tip of the peninsula contains the ruins of Catullus's (84-54 BC) villa, hot springs, and two super luxury hotels open only in the summer (Villa Cortine, rated #1 by the Cadogan guide, (030) 990 5890, fax 916 390; Grand Hotel Terme, (030) 916 261, fax 916 568).
Moving clockwise around the lake, one comes to Salò where Mussolini set up government on one end of the bay and his beautiful mistress at the other end. The next stop is Gardone Riviera, a more glamorous spot containing the winter home of the Austrian Emperor (Villa Alba) and the Italian equivalent of the Hearst Castle: Villa il Vittoriale. The latter was built by Mussolini for Gabriele d'Annunzio a poet who became a national hero after troops on a private invasion of Fiume, a town promised to Italy after World War I but given to Yugoslavia. Read the Cadogan guide description of the place.
Riva is notable for its castle, nearby waterfall, and generally fine center. Malcésine has a cable car that goes to the top of Monte Baldo for a view of the Dolomites and the whole lake (or of a whole cloud during the three weeks I spent in Northern Italy in winter) and will be appreciated by German literature fans. Goethe was imprisoned here as a spy in 1788.
Lake Garda is almost completedly booked during July and August so plan ahead. The lake is also very good for windsurfing and sailing. It seems like a very pleasant place to spend three days in the summer. The Eyewitness guide to Venice has a fairly good section on Lake Garda.
N.B.: A lot of folks apparently feel that Garda is a good place to spend a long, long, time...