If you're not staying at one of the downtown hotels (HIGHLY RECOMMENDED to stay down there), there's a day parking lot by the park on the West side. It's about 2 blocks from the convention center, costs about $10-15, and there's generally plenty of parking. On the downside, I don't think you can park there overnight.
There's also mall parking, though I don't know how much that costs or whether or not you can park overnight.