In reply to Bobling:
living round the corner. The main thing about Obertraun is caves, 3 of which which have been equipped for tourists with guided tours. The ice cave is a must-see, Mammuthöhle and koppenbrüllerhöhle are the other options.
If you go to bergfex.at, search for obertraun, you are invited to open the Austrian topo map 1:50.000 with all the marked trails availlable. Depending on the fitness of the party, you might traverse the Dachstein plateau to Simony hut (5 hours round trip), move up to Taubenkogel (some scree) for wonderful views, or do some shorter walks.
Hallstatt and it salt mine, and the mountain, Plassen, above it, are another option. Don't leave without seeing the Beinhaus in Hallstatt. Your son will be impressed.
Easier and more open walks are to be had around Bad Aussee & Grundlsee.
Feel free to contact me. Per pn if you want some more out of the way suggestions.