One of the first things you should know if you want to do this, is that many of the big parks actually have more than one stamping cancellation spot - and each spot has a separate cancellation stamp. This means that you could get 2, or three, or even more different stamps at one park. For example, when we were in Yosemite earlier this year, I got stamps at the main visitors center in Yosemite Valley, Big Oak Flat, Wawona Hotel, and Happy Isles Nature Center. We didn't get to Hetch Hetchy, and we couldn't find the Tuolumne Meadows spot.
Another thing to keep in mind if you do the passport program is that only the national parks, monuments, etc. operated by the National Park Service (NPS) are officially part of this program. I found this out because as we were driving through Montana, I saw a National Landmark sign for Pompeys Pillar. I made John make a split-second decision to exit the freeway and stop.
Bureau of Land Management - not the NPS. So, long story short, just because you see a highway/freeway sign for a National Monument, etc., it might not actually be a part of the passport program.
On our mammoth two-week road trip in May 2012, I was able to get stamps at four different NPS sites, as well as Pompeys Pillar. Again, as I've mentioned before, I got my four stamps at Yosemite and one stamp at Devils Tower. I also got the stamp at the John Muir Historical Site.
Painted Canyon site in Theodore Roosevelt National Park (TRNP) in western North Dakota. There are other stamp sites in TRNP and I'm hoping to get those on another trip.