In toy or discount stores there are inflatable boxing gloves called Sock'em Boppers. When inflated they are about 12" in diameter. Lie on top of the partially inflated tube and insert your penis where your hand should go; you can roll around and thrust to your heart's content. They're inexpensive and much easier to use and conceal than an inflatable doll, but they work really well. If you have the patience, you can fill the Sock'em Bopper with warm water. Non-oil-based lube is recommended.
WITH A CONDOM
This involves putting on a partially inflated condom. Blow up the condom very large — when you try to put it on, most of the air will rush out, but that's okay as long as some air is left in it. Once you have the partially blown-up condom on your penis, let out enough air until it's inflated only to the normal shape of the condom. (Keep too much air in it and the condom will pop. "Ribbed" condoms feel better with this technique. When you're done, just throw out the condom.