If you are fit and wearing good hiking shoes try climbing Mt Lofty. Great scenic walk. Popular with tourists and locals. Great views from Mt Lofty summit of Adelaide.
Great spot to see the skyline of Adelaide from a distance. Cafe at the summit is decent except a little on the pricey side but understandable given the location. Well worth doing some of the hiking trails that surround the summit.
A 25 minute drive will get you to the top of Mount Lofty where you can overlook a large portion of Adelaide. There's often kangaroos spotted in the nearby bush too! There is a beautiful cafe/restaurant at the summit and Botanic Gardens nearby to enjoy also.