Roadside attractions between Rapid City and Denver.

Some good recommendations above, and going through Nebraska is a better way to go than going north on I25 from Denver into Wyoming, then up to near Douglas WY. The drive from there on US 18/20 to Lusk and then north on US 18/85 is pretty boring, unless you enjoy looking at 5mile long coal trains! It's wide open empty and very windy. There's a haunting beauty about the wide open plains, but after a while it does get tedious. AND gas stations are few and far between!
Yup, better to go through Nebraska!

the archaeology site is called the The Hudson-Meng Bison Kill site...just search it on the net for more info. The Toadstools area has a campground, but primitive, with a walk laid out through the formations and the trail goes all the way over to the Bison kill site if you like a longer hike...about 3? miles or less. Bathrooms at the kill site and a rough campground there, too if it is open.

you'll need a map to get out there because the signs are pretty easy to miss.
or you can google earth it and see the course to take.

there is a surprise steak house right out in the middle of nowhere, with very good steak dinner at not too bad prices.. called High Plains homestead. you'll see pamphlets at the state parks.
On June 6-8 this year, the plains indians are having an intertribal "fiesta" at the park if you're interested.

Thanks for the tips guys. I should mention that I am after geology and ecology stuff like the toadstool park mentioned. That's right up my alley. I also found Chimney Rock and Crescent Lake National Wildlife Refuge as potential stops.

If you like history and quiet, check out Ft. Robinson S.P. at Crawford. Peaceful for hiking, horseback riding, camping, etc. Crawford is about 20 miles west of Chadron.

Other than that, go to Sidney which has a big Cabela's and Cabela's has a nice campground. Across the street from cg. is Mi Ranchito, a great little cafe with Chihuahua style mexican food.

Alliance has one or two little rv parks, nowadays and they looked pretty nice a couple years ago when we went through.
I've been told there are little parks in the little towns spread along highway 2, where you can overnight, etc. but have not explored them. Over at Halsey is a nice state park that is home of the NE State tree nursery.

North of Alliance is Chadron State Park which is a very nice state park.
West of Chadron is Ft. Robinson State park with lots of history and a little dinosaur museum and there's an archaelogy site about 20 miles north of the town of Crawford in some interesting chalky terrain called the Toadstools.