Skeletons! The Museum of Osteology is in Oklahoma City and is adjacent to Skulls Unlimited International, the commercial venture that supplies skeletons the world over. The museum opened in 2010.
It's an amazing place; much more informative than your typical natural history museum, which often have preserved (stuffed) animals in some sort of natural setting.
But, when you can see (literally) the under-foundations of all these animals, the similarities and common parts of all of them is really apparent. It almost seems that if you could stretch and shape individual bones, you could create pretty much any other animal--the basics are all shared.
Seeing that whales have "legs" is always astonishing.