Mount Gambier is named after the volcano it's built on - now isn't that an introduction? Mt Gambier is 45km from Adelaide, and is only 17km from the state of Victoria, so hiring a car here makes for a good exploring experience. There are three craters on top of the volcano, which are filled with water - the volcano also has a couple of cave complexes which are great to visit. Mount Gambier is one of the most populated cities in southern Australia, so it's worth hiring a car to make the most of the shopping opportunities and cultural sightseeing.
Mount Gambier is famous for its volcano, and in particular for the Blue Lake. There are three lakes on top of the volcano, but the Blue Lake is special because it changes colour every year from blue to bright turquoise. The lake supplies water to the city. Although it may seem like visitors have all the attractions they need at the volcano, hiring a car in Mt Gambier is still a good idea - Mt Gambier has a lot of parks, museums, and restaurants to enjoy. There are also a number of organised self-drive tours, which you can only enjoy by renting a car.
The opportunities for day trips out from Mount Gambier by car are endless - the city is between Adelaide and Melbourne - although neither are particularly close, it is possible to make the trip last a few days whilst sight see along the way. It is a possible stop off for people returning from Kangaroo Island, a trip which could be done the other way around. There are lots of caves in the area, many of which can be explored with a guided tour, or a self-guided tour if you decide to explore the Wet Cave, which like many caves in Mt Gambier, has amazing rock formations, some of which contain fossils.