Ciclabile Dobbiaco Lienz
Classic Pustertal route, suitable for everyone. The bike path is well marked and it is easy to follow, in summer time you have to pay attention to the large presence of tourists. The bottom is always paved, so it is doable with any bike. As you can see from the photo it is not very wide so be careful, although it is difficult to cross people in the opposite direction since most face it downhill, arriving in Lienz.
For the return to Toblach you can either choose to use the train, the Lienz station is right at the end of the bike path, or turn to one of the many shuttle services. In the latter case, it is best to leave having already booked the ride.
Last tip: Don't miss the Loacker store in Panzendorf; expect a large number of people here, too, however.