This is one of America's great tourist traps. Wall Drug was founded in 1931. In 1936 they put up a sign on the highway for travelers headed to see Mount Rushmore. It said "Free Ice Water". The story goes that they've been busy selling ice cream (it was great) and giving out ice water ever since. You can also buy nearly anything else, and you can see some freaks of nature. From mounted heads of the Western Jackalope, to a six foot rabbit, Wall Drug has it all.
The other reason to stay in Wall is that it's about 8 miles to the west gate of Badlands National Park. The Badlands are a wild land of rugged, beautiful hills formed by millions of years of erosion. As best geologists can tell, this area was an ocean about 65 million years ago. They've found fossils of clams and other marine life. Since then the area has collected volcanic ash from distant eruptions (possibly from Yellowstone?). The result is the radical layers of color you can see in the resulting hills.
The visitors center says that Badlands National Park has the richest collection of prehistoric fossils in the world. The yellow color they say results from the vegetation that grew shortly after the oceans receded. The reds and tans are from more recent times (only over the last half a million years or so).
Mostly though, it's a surreal place to see in person, and like Yellowstone, it's extremely hard to capture the magnitude of the park with these pictures. For reference, it was hot-in the low 90s. And, it was windy-consistent gusts of around 20mph. The result was a very strange whooshing noise as the wind whipped through the hills and valleys.
The most interesting part of the Badlands is that it's constantly eroding. The area is arid, and gets 14 inches of rain a year on average. The hills are dry and changing constantly as a result of wind and rain. It will certainly take a long time, but experts predict that the landscape we see today will someday be gone altogether.