We are staying in Lake Garda for 3.5 days. I am only interested in things to do on Lake Garda. We've been to Milan, Rome, Verona, Venice, Cinque Terra, etc. (we live in Germany) but we've never been to Lake Garda. We are staying in Riva del Garda - please offer some day trips. WE have a car.
YOu may not need or want to day trip, Riva del Garda is one of the larger spots on the lake and rather interesting on its own.
However there are several spots that might be of interest to you. You can google them to get some ideas.
on the eastern shore:
Malcesine, Torri del Benaco, Bardolino
On the western shore:
Limone sul Garda, Tremosine, Gargnano, Salo
Sirmione at the opposite end from you is also interesting but farthest away.
You'll have to decide whether to drive or use the ferries.