Hobart is a great little city with plenty to do. A short trip up to mount Wellington is a must, the botanical gardens are great, and if you're appreciative of art, the museum of old & new art (MONA) is one of the best art galleries in australia, in my opinion. Some may deem a few of the installations/ images a bit too explicit for kids, but it depends on their age I suppose. You can try one of their beaut beers 'moo brew' at the gallery bar/ restaurant. You can get to MONA via a boat cruise up the Derwent river from the wharf, or drive. Hobart has some great food on offer, particularly around the wharf, Salamanca and north Hobart. The museum is not bad either, and I think it's free admission. There are the obvious tourist haunts such as the weekend Salamanca markets (Saturday only for memory), but personally I'm not a fan. If you have a car, head down Port Arthur way and stop at the tessellated pavement and devils kitchen (a blowhole), and you can sample some awesome Tassie oysters! Wineglass bay in freycinet NP is another beaut spot, it's about a 2.5 hour drive from Hobart. A closer and interesting day trip worth a look is Bruny Island. Google some of these suggestions...
If you're after herping activities, I'm not too sure where to go, but a short drive out towards Huonville on dusk and you might spot wombats, wallabies, bettongs, quolls and possibly a devil - although the devil population on the Tassie east coast has been severely decimated due to tumor disease. Enjoy!