Anywhere in that region would be nice; Radolfzell would be good. I've stayed at the
Krone in the centre of town and can recommend it – traditional and nice. You can get a boat or train to Mainau from there. I don't think there are any hotels on Mainau itself (it's an island). From Radolfzell it's also quick and easy to get to Konstanz, or to the other jewel of the region, namely
Meersburg.