Although they are all just rodents to me - I'd vote for guinea pigs. After growing up with my brothers and sister - and then my girls keeping a variety of rodents - I think guinea pigs are about the easiest to keep - and they very rarely bite. They also like to be held - they don't normally try to escape, run off and hide causing extended searches for them. The are night dwellers - so they will be active when you are both home then. They like the exercise wheels and balls - not so much the tube things from what I have noticed. Keep is cheap with the food and bedding (buy the really large bales).
Any small animal will smell if you don't keep the cage clean. For guinea pigs make sure to add extra chips, shavings, bedding whatever so they have a place to make their nest and one end for their business. As noted an inexpensive aquarium works the best and keeps the mess confined - although it does keep the smells from dissipating.
How about turtles? I've got a pair here looking for a good home -
Ron