I've kept hermit crabs in 10g tanks. Most recently I had a few in a tank with gravel substrate and a container of sand and a shallow container of water and a wire top on the whole thing. Oh, yeah, and don't forget things to climb on! I was considering a rock type reptile heater and instruments to measure temp and humidity and maybe a mister. I have a tank now that I've thought about putting hermies in again, but my wire top got old and I don't want stuff crummbling off and falling in, so I might use the flourescent light hood that's on the tank, but turing on the light might dry them up.
My best crab was the one I had when i was a kid for about 5 years. I called her Shelly. She lived in a fishbowl type glass candy bowl that usually didn't even have a top on it, so if her climbing "trees" were too close to the round whole at the top she might escape, which really freeked out the house sitter once. (imagine a little girl's pet scaring a big man! He scoopped her up on a piece of paper so he wouldn't have to touch her.) Anyway, we just poured a glass of water in to keep her gravel moist and she usually did fine, but Austin was probably a lot more humid that Sacramento. She finally died from drying out right before we moved because she was living right under a lamp and maybe didn't get enough water or something for a little too long. I've never had another crab last longer than about a year, but Shelly was wonderful. I wish I could get another one just like her. She loved to try on new shells and once we even took her clothes shopping at Michaels and let her choose her own new clothes right there at the store. She rode in my mom's purse and the lady at the store actually kind of liked watching the critter crawl around and explore a selection of sea shells. She liked murexes best, which was cool with me, because I liked to tie a shoe string to the spiky end that sticks out and take her for walks on her leash.
We've also kept rodents in 10g aquariums, especially leakers, which are sometimes even suitable for crabs.
I haven't had a reptile or amphibian yet, but I'm considering it.