Jennifer,
I doubt that it's going to be possible to see much of Lake Garda in a day, especially as you'll need two hours travel each day to get there. I don't know of any guided tours for the lake, although there certainly could be some. I'm not sure I'd describe as a "fun place to visit" (that is unless you'll be visiting Gardaland - https://www.gardaland.it/en/# ) however it is a scenic place to visit.
It would be possible to see a couple of the cities along the lake by Ferry. If you decided to do that, I'd suggest taking the train to Peschiera del Garda as the ferry dock is quite close to the rail station. It's about a 15 minute walk from the rail station, or you could take a cab. Two cities along the lake you might consider are Malcesine and/or Limone sul Garda. Try to use the fast boats (Servizio Rapido) so that you minimize travel times and maximize sightseeing time. They do cost a bit more but that's a better use of your very limited time. Here's a link to the schedules - http://www.navigazionelaghi.it/doc/pdf/orari/Garda/BATTELLI%20primavera%202019.pdf .