I turned my Grandkids into amateur geologists and amuse them at the same time by way of the
"Roadside Geology" series of books. There's one for each of the fifty States. It's also a great way to get them to actually LOOK at the landscape they're passing through. A few small digging tools and a little canvas bag for each goes along, too. (The bag size limits how many "specimens" they can bring home).
Of course, one must take care to check for permissions if collecting- usually unrestricted along roadsides, but not allowed in some National Park areas etc.
