The Dinosaurs and Fossils Gallery of the Science Museum of Minnesota occupies a more-than-respectable 10,000 square feet of floor space, and boasts (among other specimens) one of the most complete Triceratops skeletons in North America, as well as the largest Camptosaurus yet discovered. This wing also features a Diplodocus (the remains of which were discovered by Minnesota high-school students), a Stegosaurus, and an animatronic set of T. Rex jaws.
Once you're done with the dinosaurs, there are plenty of other things to see at the Science Museum of Minnesota. "Primeval Swamp" focuses on the crocodiles that occupied the ecological niche of dinosaurs after they went extinct 65 million years ago, and features both real crocodile fossils and interactive displays. You can also see some of the prehistoric mammals that inhabited ancient Minnesota and North Dakota, including a saber-toothed tiger and a North American camel.

