Theodore Roosevelt National Park is the single best day trip from Williston, though at roughly 200 km southeast via US-85 it's a long day. The North Unit of the park is the closer of the two and is worth the drive on its own — dramatic badlands scenery carved by the Little Missouri River, bison herds, and a quality of solitude that the more famous western parks cannot offer. Allow at least four hours in the park itself. The South Unit, near Medora, adds another hour each way and is better saved for a separate overnight.
Lake Sakakawea State Park lies about 160 km southeast of Williston, where the Garrison Dam impounds the Missouri into one of the largest man-made reservoirs in the United States. Boating, fishing for walleye and northern pike, and camping on the bluff above the water are the draws. The park is busy in summer; the lake's scale — over 300 km long — means there is always somewhere to find a quiet cove.
Crosby and the Divide County Badlands offer a lower-key alternative for those who have already done the Roosevelt National Park circuit. The farming communities of extreme northwestern North Dakota sit in a quietly dramatic landscape of prairie coulees and native grasslands. Crosby is about 100 km north of Williston and has a small museum with exhibits on the regional homesteading history.