I like the landscape of the north lake the best.
On the north lake I like Limone Sul Garda, Malcesine, Riva del Garda, Torbole, Gargnano, Torri, etc. They are all very small, except for Riva which is a bit bigger town, although the historic center is also very quaint.
On the south side you have two large resort towns: Peschiera and Desenzano. Peschiera has a famous fortress and it has historic significance because of events taking place during the Italian Risorgimento (1800s), when Italians fought to gain independence from their oppressors of the time (in that area it was the Austrian Empire). Italy was continually invaded by someone throughout its history, nowadays it is the tourists.
Between Desenzano and Peschiera there is the pretty medieval town of Sirmione. Sirmione is quaint and worth a visit, however I'd like to warn you that when you go (in July) it will be crowded with hordes of tourists worse than Disneyland. I think you should visit the place, however I just wanted to alert you that everybody else around the lake will visit that town during their stay, and many will actually stay in the town itself, therefore hotels in that town are probably the most expensive. It's like the Cinque Terre. They are famous and pretty, so everybody wants to go there.
As far as beaches are concerned there are many in any area. The ones near Sirmione are probably prettier (there is one I remember is called Jamaica Beach), but you can find near any town you choose. The website below lists them all according to town. I think it's in Italian only but beach in Italian is SPIAGGIA (pl. "spiagge").
http://www.tuttogarda.it/guida_spiagge.htm