These are really fun and spark interest in the National Park Service system. The brainchild of a marketing genius, the purchaser can get a stamp from each of the NPS sites he or she visits. The collection of these stamps, similar to postal cancellation postmarks (which include the name of the park and the date visited) become fun to collect. It's a great way to get the kids (of all ages!) excited about going to different parks, monuments, seashores, etc. that are operated by the NPS.
The passport itself is reasonably priced and the stamps are, of course, free. Each NPS facility has a stamp available at the visitor's center. If you don't see it just ask the ranger on duty. Some (e.g., Mt. Rushmore, Cape Hatteras Lighthouse) have special stamps with a depiction of the area/monument. Great fun!
You can obtain an NPS passport at any park Visitor's Center or online at the National Park Service Store.
In and around Richmond, you can get stamps at the Richmond National Battlefield Park, Maggie L. Walker NHS, and Petersburg National Battlefield. Further afield you can also get stamps at the Fredricksburg & Spotsylvania County Battlefields Memorial National Memorial Park, Manassas National Battlefield Park, Appomattox Court House NHP, Jamestown NHS, Colonial National Historic Park, and the various sites in and around DC.