Nice but long hike/walk on an easy terrain, crossing several peaks and offering great view to Karwendel.

Starting from Mittelwald in direction of Hoher Kranzberg, walking up under the lift - not the best part of the route since it is steep and a bit crowded and one hears and sees the lift all the time. On the positive side - in one hour the first peak is reached and half of the height meters of the whole hike are done. 

The next part of the route follows the "Fußweg" leading to Elmau castle. There are multiple ways to get to it, most of them going on a broad mountain road. The Fußweg, on the other hand, is going through the forrest and is very nice to walk and rather unfrequented. The path goes slightly up and down the whole time and is very scenic. 

The Elmau castle  (a hotel nowadays) and its surroundings are a nice place to make a break, since it is open and flat and thus quite sunny.

After the Elmau castle the route gains some height meters again (not too many though) and leads through another forest in the direction of Eckbauer with option through Wamberg. I recommend this option because this route goes a bit higher and from it to the left there is a great view of Karwendel. Walking is here mostly on a wide mountain road. 

On Eckbauer there is a Gasthaus, which is also a nice place for a break, offering traditional fool and drinks.

The last part of the road is the descent from Eckbauer to Garmisch. Not a very nice part with quite steep asphalt road, but it doesn't take long. It can be omitted. of course, with using the cable car down. 

Another possible alternative is to descend from Eckbauer to Partnachklamm - I have not checked this part of the route but it looks steep, so maybe not so good in slippery conditions.

Summary: This route can be done all year I assume, since it goes mostly on wide paths. The only part following a path through a forrest is marked on the trees, so snow is also here not a problem for the orientation. There are usually multiple available options to connect the highlights of the route and direction signs are everywhere, so there is no danger to lose one's way. 

It can be done also in the other direction and the first and/or the last part can be skipped and done with the lifts (Hoher Kranzberg and Eckbauer).