I wouldnt recommend it, but air is probably the safest way to go if you want to give it a try out of curiousity. The first time I tried it with an electric air pump it felt amazing.
So much so that I made a habbit out of doing it (im a raging glutton) after about the eighth time I did it the pain started to outweigh the pleasure. I asked my doctor about why I kept getting cramps increasing in intensity and admitted to my fetish and he said it had something to do with the muscles in my colon becoming overdeveloped and wasnt anything serious.