taking a car from Meersburg to Mainau and into Switzerland

Cars aren't allowed on the island. There are passenger ferries, but Not Needed for you since you have a car. As you drive NW on the B31 and arrive at Meersburg there will be a turnoff that has a sign pointing to the ferry boat landing. It's a car ferry that goes over to Konstanz. Just drive down the hill and get in line to drive on the boat. When you get off the ferry take a right and you'll see a sign pointing to Mainau. It's only a 5 or 10 minute drive to the Mainau parking lot and from there you walk across a footbridge to the island.

Laura, Mainau is not far from Konstanz but it takes a little longer than five or ten minutes to get there. Konstanz is a border town, so you can cross over into Switzerland from there but you need a special decal on your car to drive in Switzerland, which is not cheap. Maybe you can get a rental that already has a decal... Konstanz has a really interesting modern fountain, which I found hilarious but my staid friend did not. It's called the Lenk Brunnen. Mainau is beautiful. It is, by the way, in the state of Baden-Wuerttemberg, not in Bavaria. Regards, Anna

fyi: The ferry dock is on the far n.e. side of Konstanz and just 2 miles to the Mainau parking area. Ergo, less than 10 minutes away.

Laura, Mainau Island is not in Bavaria, it's in Baden. Other than that: It's tiny and there aren't any roads on the island. From Meersburg you take a passenger boat to the island. Alternatively there is a car ferry from Meersburg to Konstanz-Staad. You can then park on the big car park and walk across the pedestrian bridge to Mainau island. You can take any rental car into Switzerland. Konstanz also is the border-city to Switzerland (Kreutzlingen). The car ferries between Meersburg and Konstanz-Staad run every 20 minutes and journey time, too, is 20 minutes. The fares are pretty affordable.

Andreas,

I hope we don't fall out over this, but if the OP wants to use a Swiss Autobahn their car, rental or not, will require a current Swiss vignette. I agree that many will have a vignette already but if the OP neglects to mention that they are visiting Switzerland or Lichtenstein when they get the car, or even if they do, they may find themselves shelling out CHF40 at the border, or an expensive fine if stopped on the Autobahn.

Yes, of course you do need a Swiss freeway sticker. Most car rental providers in Southern Germany have got many cars available with Swiss and/or Austrian freeway toll stickers. Mention it upon reservation or at least upon pick-up.