If one of them has a birthday, blow it up and then ice it like a cake. When they try to cut it with a knife it'll burst, covering them in icing and shit (if you shit inside first).
They'll be fucking pissed off that it's just a huge bag and not a cake.
First, call the cops and tell them that your friends have murdered someone and thrown him off a bridge into the local river. Then take the bag to said bridge with a few cement blocks. Put the blocks in the bag, climb in, close the top, and flop over into the river. When the cops drag the river and find your body, boy will your friends will have a hard time explaining that!!