The time you’d like to spend is up to you. Many Europeans, including Italians spend several weeks at lake Garda in summer every year!. It depends on how much time you have available and how much you want to devote to various places.
Yes, you can get around by bus or ferry. Some people prefer that rather than having to deal with a car and parking, which is not easy to find on some busy periods.
I like the towns on the north side because it’s more mountainous than the southern half. Basically any town from Garda (on the east shore) or Salò (on the west shore) and north, all the way to Riva, is good for me. Garda, Malcesine, Torbole, Riva, Limone are popular spots.
You can do all of the above activities, and more, like biking, paragliding, windsurfing (one of the top spots in the world), mountain climbing, swimming, mountain biking, cliff para shooting, water theme parks, auto safari with exotic animals, wine tasting. There is no other lake in Italy that has the plethora of activities that lake Garda has.
Below is the official tourism website:
https://www.visitgarda.com/en/garda_lake/
