The obvious day trip is Quartz Mountain State Park, about thirty miles north on the shore of Lake Altus-Lugert. It is the only nearby landscape with significant relief and is genuinely worth the drive — hiking, swimming, fishing, and lakeside picnicking are all on offer.
Further afield, the Wichita Mountains Wildlife Refuge (about 75 miles east, near Lawton) is the largest natural area within day-trip range — granite peaks, bison and longhorn herds, and walking trails through prairie and oak woodland. Combined with a stop in Lawton itself, it is a full day out.
Lake Texoma is about three hours east on the Oklahoma-Texas border and is the largest reservoir in the region — a popular spot for boating and fishing if you have a weekend rather than a day.
For history, the Fort Sill Museum at Fort Sill (also near Lawton) covers the Indian Wars era and the post-Civil-War frontier army with depth and seriousness.