Montana's Yellowstone Country is giving away a free Ranger Pack that includes a sticker sheet as well as a travel pack and seasonal travel information. It looks like there's around 12 cute stickers on the sheet.
If you're thinking about roadtripping up there at some point, then you might also be interested in all the free state maps you can get.
My 7 year old grandson would love this packet of all things nature & Yellowstone – free Ranger Pack