Walk around the lake with beautifull views on the lake and Hallstatt.
On the last part of the route between Hallstatt and Obertraun you have to follow the road. It's very safe as you are on the separated cycle track, but not very pleasant.
I suggest you take the boat from Hallstatt to railway station at other side of the lake and walk back to Obertraun. For 2.5€/person you have a small cruise included then :-)