The two places I am speaking of are just 40 mins north of Rawlins on I-80 or about the same distance southwest of Casper on I-25. Both destinations are practically free, educational, and even fun! The places I am speaking of are Independence Rock and Martin's Cove via the Mormon Handcart Vistors' Center (Devil's Gate is right nearby), both major historical landmarks on the Oregon Trail.
Martin's Cove is a historic site administered by The Church of Jesus Christ of Latter-day Saints. Martin's Cove is an important part of Latter-day Saint history which is why the Church acquired land there near the site to run a visitors center and other facilities. Martin's Cove itself is BLM land so there is no proselyting there for those who might be concerned about that. (By the way, I am a Latter-day Saint.) Entrance to the Mormon Handcart Visitors' Center is free. The tours are very flexible and given by missionaries for the LDS Church. The missionaries there were nice enough to take us to the Cove (3 miles from Visitors' Center) on a Mule (not the animal) although we cold have done the hike with a handcart if we had wanted too. There are lots of different buildings to check out at the Visitors Center as well as clean public restrooms and a nice picnic area. If you are tight on time, like we were, just let the missionaries know what you would be interested in doing, and they'll do their best to make it happen.