There are tons roadside rock & fossil shops all over CO, NE, WY, UT etc and growing up I used to hunt & collect for both a fair amount (also collected seashells). I had probably 5 or 6 large boxes full of stuff and I can't remember what happened to it all...

Some of things I've personally found over the years...
An Arapaho or Cheyenne arrowhead at pretty high elevation; a couple of ancient campfire sites; a shard of pottery; a ton of various plant fossils; a really large clam fossil that I left in place; a late 1800's Laudanum bottle in trolling Shilshole.

I also found a great piece of Obsidian buried deep during some excavation work in my current yard. It's all mixed glacial till where I'm at, so either a glacier brought it here, an ancient native dropped it here or it blew outta the top of a distant volcano. All I know is it shouldn't be found here.