Items you own
If you own the item and it’s not a living sentient being such as a dog or a snake, however in some states beastiality laws are lax and you can shove animals up your ass, you can shove it up your ass. I don’t rule out living things because plants are alive and you are legally allowed to but them up your ass. So pretty much anything you’ve cultivated yourself whether it’s through money or accruement of things you found on your path, you’re legally allowed to stick it where the sun don’t shine
Things you find
Again, if you’ve stumbled upon it, and it’s an inanimate, unliving object, you can do whatever you want with it. If it does not look like it has an owner that misses it, such as a nice Rolex or a cuckoo clock, you can shove it in there. Rocks, shells, discarded pieces of nature or trash, up the 2 hole and you won’t hear a peep from Johnny law.
Living things with consent
Human appendages, someone else’s plants, the opportunities are endless when it comes to living things that are consenting to being shoved up your bottom side.
If someone is willing to put something in there, just make sure you say “hey, so this is something you’re down to do or at least are willing to try because I want to?” And once they say yes, you know, you can do the deed.