ISpeakForTheSeas said:
Full disclosure, I don't have a mantis shrimp (at least, not yet - I might in the future), but for most animals, I look at cat and bird toys for some inspiration on how to provide them with some good, quality enrichment. So, my first thought would be some kind of food puzzle toy - though, I'm not sure what kind of food puzzle would work well for a mantis shrimp. Maybe the whiffle ball kind where the food is in the center and falls out of the holes when knocked around properly (you'd need a very durable &/or flexible ball for this, though, as it would likely be punched/speared repeatedly). If the food puzzle works or you want to try a different kind, there are plenty out there, and you can be as eccentric or mundane with them as you want.
Another thing you could try (I have no idea if the mantis would go for it or not) would be one of those bell/rattle ball toys (obviously you'd want one with no metal that can take a fair amount of abuse).
Also, while blaxsun's comment above was clearly intended to be humorous, you could actually try it in a variety of forms, and it might actually work. You could hang the "punching bag" in the tank a ways away from the walls and have it be either a rattle toy that sounds when "punched" or a food puzzle toy (you could do something like a mesh bag with food inside that either cuts open when speared, or one with wide enough holes that some food comes out when it's "punched" - you'd want to keep an eye on any mesh though, just to make sure the mantis doesn't get stuck, so this would be a "with supervision" toy until you've tested it enough to feel it's safe). Or, you could even have it be both a rattle and food puzzle (just put the rattle portion at the very top or very bottom would be my suggestion).
Again, not having a mantis, I have no idea if they would go for any of these toys or not, but I'd be interested to know if you decide to try any of them.
