I know it's a big call saying that Adelaide Central Market is the best market in Australia but I reckon it's true. Melbourne's Victoria Market come a close second, but for fresh food, atmosphere and accessibility, Adelaide wins. There's such a variety of food stalls there; one of the key positives is that they're all so close together, adding to the sense of it being a real 'market'.
Many places offer free tastings too, which means you can try before you buy - always a bonus. There's even a great (and really cheap) food court that offers the best that Asia has to offer. If you can drag yourself away from the smells and sights of the main market, that is.
It's also handily located in the middle of town, so that, regardless of where you're staying in the CBD, it should take no longer than 15 minutes to walk to it. I reckon the Central Market is cheap too; Melbourne's Victoria Market is a bit more expensive, plus it's stuck up one end of town which doesn't make it central, either.
The Central Market also has guided tours, which gives a behind the scenes insight into the workings, history and goings on. I would heartily recommend that, even if you only have a couple of hours to kill, the Adelaide Central Market should be a destination you visit when in town.