I recently got some African Pygmy Mice. They are pretty amazing. Pygmy mice are the smallest mouse species, and possibly the smallest rodent. I am keeping mine in a 10 gallon aquarium with plenty of hiding places, it is recommended that they have one gallon of aquarium space per mouse. They eat small seeds like in a finch or parakeet mix, and I also offer them things like dried fruits, gerbil food mix, and cat food for protein. They also need a water bottle that doesn't have any balls in the sipper tube like regular small animal bottles, because they aren't big enough to move the balls. There are glass water bottles sold that don't have any balls, or the bottles sold for Crittertrail cages don't have balls, although you could never keep a pygmy mouse in one of those cages, they would easily escape. Since they are originally desert animals they don't smell, like gerbils, also they need relatively warm temperatures, ideally above 70F, and they will die if it drops below 60F.

Here is their cage as I have it now.

This is what they look like, they weigh about 1/8 of an ounce and are 3-4 inches in length, including the tail.

I put the smallest size Silent Spinner wheel (4.5 inches) in their cage, and they seem to enjoy running on it, even though they seem like they could barely make it move. I took a short video of them in it.