Day Trip on Lake Garda from Verona

Peschiera comes first from Verona, so it is closer, and there is a fortress to see. There are also buses and taxis that can take you to nearby Sirmione, so no need to go all the way to the dock for just one stop with the ferry.
If you have all day, maybe, if you start early, you could take a ferry from Sirmione or Peschiera, all the way up north to Malcesine or Riva or Limone (or all three). Then you can take the bus back from either to Verona in the evening. Actually I would probably start from the farthest locations first (Malcesine for example), by bus, then take the ferry in the afternoon down to Sirmione. Once in Sirmione you can stay in town until late evening, because you can always find a taxi to the Peschiera and Desenzano station, and trains to Verona run till very late.

These are the info resources.
Buses:
http://www.lagodigarda.it/Linee-autobus/1158-1.html
http://www.visitgarda.com/en/Bus-timetables/

Ferry info:
http://www.navigazionelaghi.it/eng/

Trains:
http://www.trenitalia.com/

Sirmione is heavily touristed and, in my opinion, rather trashy. The best part of Garda is further north. If you must day-trip, returning to verona in the evening, I'd take the train to Desenzano, then the bus to Sirmione, and then ride a ferry to Bardolino for late lunch or dinner before returning by bus from there.

If you could spend the night, take it from me, right the long Ferry ride all the way to the top of Garda, to Riva del Garda, and stay on the west end of Riva del garda overnight at (if it's affordable to you) the Bellevue hotel. Absolutely fabulous and very romantic! Many good restaurants around there for dinner, too.

some people really like sirmione. no doubt it's crowded during high season but it's also quite lovely and charming. the ruins are very interesting and it's a lovely walk from the center of town to the tip.

you can walk from the train station in peschiera to the ferries, but if i recall correctly for part of it it's not the most beautiful scenery. not like factories or anything, but nothing spectacular. check google maps satellite for more info on that. the town of peschiera itself is not as small and charming as many of the other spots along the lake (bigger, more commercial), so if you can minimize your time there you'll probably enjoy yourself more.

you could also check the ferry schedule and see what makes sense based on your timing. to go from peschiera to riva is over 2 hours. the scenery is beautiful but if you're there on a day trip it might be too much time. or, if you have a copy of the ferry schedule, you could just hop off and on whenever you saw a town you liked. :-) during low season the ferries don't run that frequently but while you're there it could work.

try to go to basically anywhere on Southern lake garda, peschiera for example, and just get on the ferry. Take the ferry (the views from the water make this the best way to see Garda) to Bardolino or Malcesine, a wonderful town with a great castle too, then take the bus home. You will be rewarded!!

Thank you all for your suggestions!

Last May I took a bus from Verona to Sirmione. It was a very enjoyable day. I wouldn't try to see more than one place. I chose Sirmione because I was able to take the one bus directly there. You catch the bus at the Verona train station.