ok, here's mine!
-you need a sandy bottom for this. take your fins off, deflate your bcd and do a back flip/front flip!
-bubble rings
while following my buddy i entertained myself by swimming above her and catching her bubbles in my hands, then going up to her as if i've just caught something amazing, when i open my hands the bubbles come out
lame but silly haha
also if you flick the bubbles they make a nice little 'pop' noise, the smaller the bubble, the higher pitched. i once popped a dugong fart! proudest dive moment ever haha
also if you position yourself just above your buddy, turn upside down, and then slowly lower yourself so you come into their field of vision with your face about an inch from theirs it scares the bee-jeesus out of them
great for when someones surveying something and has to stay still looking at a particular patch for a while
umm also just wearing novelty clothes. had a friend that always dived wearing a tophat, it was modified so he could tuck the rim under his goggle strap.
some other friends found a massive flat rock and decided to take a full dining set down with them and pretend they were having a meal