Amy, just because YOU don't know the area well enough to find your way around via public transportation, yes, trains and buses, doesn't mean it can't be done.
Here is a map of the bus routes in Oberbayern. (It's a big file, 2.5 MB).
Yes, I'm sure there are a few places you can't get to by public transportation, but first figure out what you really want to see, then look for how to get there. I think you will find most places are accessible. There are few train routes without hourly, at least every other hour, service. I'm sure you'll find ways to get there. Renting a car is a pain: it costs a lot more, you have to worry about the liability, you have to drive it yourself, you can get lost, you have to look at the road instead of the scenery.