Lake Garda / Lake Como - Two nights.

Rick's Italy book has all the info you need for the mid-lake area of Lake Como, including Varenna, Bellagio, and Menaggio. He doesn't cover Lake Garda, but you can find previous forum discussion using the search function above. Ferry schedules for both are at http://www.navigazionelaghi.it/eng/index.asp.

This website has the Ferry schedules. This is the link for Lake Como but on the page you'll also see Lake Garda link on the lefthand side.
http://www.navigazionelaghi.it/eng/c_orari.asp

I like Varenna and others on here post about it on Lake Como. It's a easy 1 hr train ride from Milan. You are in the mid lake area and a easy ferry to Bellagio. You can also take a train to Como on the southern portion of the lake.

Garda is on my list to get to (hopefully next year since this year I picked Lake Maggiore). Garda looks amazing from pics and some information. Roberto a frequent user on here mentions Limone a lot on Lake Garda, and across it is Malcesine. To the south is Sirmione. You can google all 3 and see pics. Only issue is without a car you will have to use the Ferry (or bus) to those 3 towns. The train stops at the bottom left and bottom right corners of the lake at Pescheria and Desenzano del Garda. I've heard the water sports are better over on Lake Garda. maybe Roberto will chime in with more info for you on that one.

Having been to both, we much preferred Lago di Como and understand why Rick recommends it: easy by public transportation, stunningly beautiful, relaxing.