The Tahune Air Walkway is well worth a visit. It is a suspended walkway through the tree tops of a beautiful forest and overlooks the Huon and Picton Rivers, the Hartz Mountains, Picton Valley and a spectacular forest canopy.
It is about a 1.5 hour drive from Hobart, past Geeveston, and the drive itself is worthwhile. If you are driving, you can take all day and do the Hastings thermal pools and caves as well. There are a couple of companies that do all day trips, with about a $100 price tag.

The walk itself is quite short, an only takes about 20 mintues to do, but I recommend you take some lunch, and make use of the great facilities there for a BBQ or packed lunch. The visitors centre there caters for lunch (and beers!) at reasonable prices.