Lake Constance (Bodensee in German), the third largest lake in Europe, is shared by Germany, Switzerland, and Austria. It's a beautiful lake, where the Rheine river runs through. It consists of three parts: the upper part, the lower part, and the part connecting the two by the Rheine river.
Fun fact - the lake is the only part in Europe where borders don't exist, because there is no legal agreement of where the borders lie between the three countries.
Due to its size, location, and beauty, the lake is a popular tourist destination in the area. Cities like Konstanz, Überlingen, Meersburg, Friedrichshafen, Lindau and Bregenz are all popular. From Konstanz to Meersburg, there is a ferry you can take your car on (or bike), and the view is wonderful. Not that you can plan this, but if you time your ride on the ferry for at night with a thunderstorm in the distance, the view is amazing! Sadly, I was unable to capture a photo that did it justice the last time I crossed the lake.
Some other tourist attractions in the area are the Rhine Falls, which I mentioned in my post on Schaffhausen. There are also a couple islands, Mainau, Reichenau (recognized as UNESCO heritage), and Lindau. Of course, there are several castles that you can tour. If taking the ferry to Meersburg, I recommend touring the Meersburg castle where you will get a new appreciation for central heating and modern decor. If the weather is nice, rent a bike down to Hagnau am Bodensee, which takes you right along the lake. If warm enough (fingers crossed) you can jump in anywhere for a quick swim. I would recommend water shoes if you are not used to swimming in lakes. Although completely clean and safe, there is grass growing in the lake that even I have a hard time swimming in without water shoes.
I guess I should mention that you probably want to bring your passport with you. Regardless of where you are coming from, it is easy to suddenly find yourself in a new country. This part of Europe definitely let's its borders jump from one side of the Rheine river and Lake Constance without a care. You probably won't be stopped at the border control, but better safe than sorry!