The possibilities are tremendous! Are you interested in one or a few specimen pets, or more of a habitat setup with lots of smaller critters?
If I had a cracked 75 lying around, I'd probably set it up as a temperate savannah/vernal wetland habitat with tiger salamanders and toads. You could create some tunnels along the front glass to view the animals while they're laying low, provide a pool area for the toads to spawn in, and landscape it with all sorts of neat native plants and lichens, tree stumps, and rocks. Leave the top open to give the taller plants room to grow and suspend some powerful lights over it. This tank would go through a lot of water, but it would be worth it!