Two of the biggest attractions we saw at Yellowstone were the Petrified Tree and Tower Falls. It seems kind of silly to park in a little lot, climb a path, and take a photo next to an preserved tree, but it's also awe-inspiring to be so close to something so ancient and rare. It evoked a feeling in me that was a cross between putting my foot into a 40,000 year old footprint in Australia and watching the mosquito scene in Jurassic Park.
Tower falls was decidedly more touristy, with its paved viewing platform and proximity to a gift shop.
National park gift shops are a treasure trove for odd items with "Yellowstone" printed on the side. You have your usual mugs, aprons, magnets, but also artwork like this, titled "A Mother's Pride":
A Yellowstone magnet did make its way onto our fridge.
I think I may have already used the photo below, but I like it so I'm putting it in again.