Free, ample, paved, and tailgating too. A great deal!
Follow the signs to "Sioux Falls Stadium" if the park isn't obvious enough. With major makeover in 2000, the ballpark is much easier to see, especially with all the skyboxes.
There is parking on ALL sides, including to the north (3rd base side, main entrance) and west (3rd base side near home plate, 1st base side and right field entrances).
In the (very) unlikely occurrence of full parking lots there are enough free parking on streets to handle the overflow. If you stay at the nearby motels you might as well walk the couple blocks.
Tailgating outside the Bird Cage has been pretty good in Sioux Falls in past years. Games bring out some of the most hearty partiers in the league. Even before my trip in '97 I had several reports of the friendly folks welcoming visitors to their grills and beer.
When you went, you'd look for Brian (Wollman). He was the webmaster of the "Welcome to the Bird Cage," a great Canaries fan and welcomed fans from all teams. But Brian got a great job promotion and moved to Cincinnati.