my daughter has a few rescue rats, if they are handled enough, they won't bite, she has balls for them to run in and they love it, two of them when she opens their doors hop onto the floor and right into the balls, she also gives each of them time out of the ball, usually one or two are on the couch playing while she watches tv. we buy the largest one there is, same size for my chinchilla
We placed a brick (much better than a rock) under their water bottle to file their nails when they drink, we also watch them wipe their faces on it after they've had a sticky treat and their little paws just can't clean enough.
rats are very clean inside their cages, usually pick one corner to go on and my daughter has had great success in litter training them. She uses the corner "beds" that attach to the bars on the cage and puts a wee bit of litter in there to start.