Marbled crays are by far your best bet. They self procreate, are less aggressive than most other species and pop off a new batch every two or three weeks once they get established.
Keep you substrate to just a thin layer of ideally gravel because they produce a lot of waste and you will need to do a lot of gravel vacuuming (I may glue a layer of substrate to my next cray tank that I set up to make maintenance simple). They also enjoy climbing on plastic plants and it gives them more space to live. They like shelter of some sort and the easiest thing for that is short sections of 3/4" or 1" plumbing pipe cut to about 4" lengths.
Food, they are garbage cans, I feed mine cucumber, lettuce, pellets, algae pellets, flake, frozen seafood, pretty much anything they can get their claws on.
If you are going to Pats place, then grab a batch or two of ramshorn snails also, that way you can kind of do double duty with the tank. The crays will eat the snails from time to time, but it isn't a total waste since it is all fish food anyways and they also clean up the scraps.