Once again I'm going with reviews, why? Because I asked for suggestions that's why!
Demyx
Make him listen to High school Musical Soundtracks
Tell him that the fire element is better
Tell him has less fans than Sora (after you told Sora he didn't have any fans)
Make him play Guitar Hero for 3 hours
Tell Demyx, Xaldin ate his sitar
Tell Demyx that his "Dance, Water, Dance" phrase is lame compared to Axel's "Burn Baby!"
After he eats dinner tell him that the meat he ate was actually fish
Paint his nails with Larxene's pink nail polish in his sleep
Playing his sitar badly in front of him
Get him a pet cat :3 (remember: cats don't like water!)
Well this one really stunk! Next I guess should be Axel...For suggestions, please don't make all of them be about dumping water on him, or pushing him into a pool, or making Demyx constantly attack him with water clones...I'm sure there's more than that...right?