This is a beautiful, easy loop on gravel path and forest roads around the Eibsee Lake.
Access is from the Eibseestraße (Eibsee Street) via the Garmisch-Partenkirchen to Grainau Bikeway
and Grainau to Eibsee
trail or you can park at the mouth of the Eibsee where there is plenty of parking and restaurants, hotel, train station, and cable car lift.
You can also take a train from the main station in Garmisch-Partenkirchen to the mouth of the Eibsee. The Eibsee Seilbahn (cable car) goes to the top of the Zugspitze, which is the highest point in Germany at 2962 m (9718 ft). You'll have to leave your bike. If you do start at the mouth of the Eibsee, you'll finish with a pretty good climb though, so keep this in mind.
From the Eibseestraße, follow the route clockwise around the lake. In the first mile, you'll climb through a forest about 150 m (500 ft) to the mouth of the Eibsee; that is the hard part. The next three miles follow the shore of the Eibsee along a gravel path. Expect a lot of hikers on this portion of the trail along the shore, so be courteous and say "Grüß Gott" (pronounced groos got; literally translated greetings from God).
As you reach the westernmost point on the loop, you'll have excellent views of the Zugspitze. When you leave the shore of the Eibsee, you've got a quarter mile of 10% climbing, but once you finish that, it's all downhill on forest roads through a dense forest back to the starting point.
If you want more, you can climb the Eibsee to Obermoos
trail into Austria to the Hochtörlehütte (High Thorn Hut) for a home cooked meal with appropriate beverage. Or if you are really adventurous, do the entire 50-mile Wettersteinrunde (Loop Around the Weather Rock)
, which circumnavigates the Zugspitze.