Hey there, as this is something I always found to be satisfying and fun, I had always wondered if anyone else did it on a regular basis.
What I like to do is grab a pair of spandex/lycra swimtrunks (ordinary compression shorts usually work too) and sit over the air jets in a hot tub/jacuzzi, the idea being that between the material becoming somewhat water tight and the air entering the trunks that they begin to inflate.
Of course, leakages tend to occur as air is pushed to the bottom and top of the trunks. To solve this I either use waterproof tape or a "liquid latex" substance to seal off the whole of the top and most of the bottom of the legs.
What also helps is to have a hose which leads from one of the jets to the inside of the trunks, which of course can create some nice stimulation...
I personally have loved this feeling and the look of that huge balloon of air in the front of my trunks, but I never knew if I was the only one who enjoyed it.
Let me know below, especially if you have any suggestions on how my model may be improved.
I don't usually do this on a regular basis, but I know it's pretty common and it's innocent enough that people don't realise it's a fetish. It's just plain fun to do sometimes.
(Not on here too often, replies might be slow.)