Next time you're having a bath, take a balloon. I originally took it for a cheap prank on my nosey family. The prank was to make it sound like I was having the mother of all farts, using the balloon. I would blow up the balloon then let all the air out under the water.
Worked great, but then I found an interesting trick. Aim the opening at your cock or asshole and let it all out. Seriously arousing. Teasing your head is a riot!