It's hard to miss Sioux Falls, as it's at the intersection of I-90 and I-29, but honestly, there's not much about the city that makes it special for a tourist. If you're planning to visit South Dakota I'd strongly suggest that you make the trip west to Rapid City and the Black Hills. Again, Rapid City itself isn't a lot unlike other small cities, but the Black Hills area makes an excellent vacation destination with Mount Rushmore, Crazy Horse Monument, Custer State Park, South Dakota Bad Lands, Devils Tower and a score of other interesting tourist attractions. Let Google be your friend and I'm sure you'll find a number of sites that you'll not want to miss.
Assuming that you do take I-90 across the state, Mitchell's Corn Palace, about 75 miles west of Sioux Falls, is an interesting short stop, as each year it has all new mosaics on the exterior made from corn. The new scenes on it are made each year in the off-season, but this year it's undergoing a major redesign so will be all new next summer.
Next (another 70 miles west) is Chamberlain, which is where you'll cross the Missouri River and in doing so enter "The West". The difference is obvious, as the land changes immediately from farmland to prairie. A good place to see this, other than just from the highway, is to make a pit stop at the Chamberlain rest area. It's on a hill next to I-90 and overlooks the river and the prairie beyond. South Dakota is commonly divided geographically as either "East River" or "West River", with East River being very much midwest and West River being very much western. It's a stark contrast with the river being the dividing line.
The next major attraction will be the Badlands National Monument, best seen in early morning or late afternoon but interesting any time of the day. You can drive through it in an hour or less, but I'd suggest taking another hour to get out of your car and take a short hike or two. Trails abound, from easy to more lengthly, but all of them will give you a little closer look at the formations.
At the western edge of the Badlands is the small town of Wall, made famous by its drug store, Wall Drug. You may have seen road signs for in in Missouri. I've seen them a thousand miles into Canada and all around the U.S. It's really just a tourist trap, but as the grand daddy of all tourist traps, it's worth a stop and is a good place to buy some souvenirs. I've purchased Black Hills Gold jewelry, boots, wallets, and trinkets there, as well as breakfast and lunch.
It's an interstate hour from there to Rapid City. I'll let someone else handle the Rapid City and Black Hills attractions, or just do a Google and ask questions that you might have.
The only other thing I'd suggest, as a Wyomingite, is that you consider continuing your trek into Wyoming as well, where you could visit Devils Tower in the northern Black Hills, then the Bighorn Mountains, especially if you haven't been to the Rockies before. While the Black Hills are beautiful, they're old mountains and top out at under 8,000 feet. The Bighorns are much more majestic with jagged peaks that reach to 13,100 feet. Buffalo and Sheridan are small, historic towns on the eastern edge of the Bighorns. Both have plenty to offer the visitor with an interest in Old West history, but be sure to drive up into the mountains. And at this point you're only a couple hours from the the nation's first and grandest national park -- Yellowstone!