The best replacement for sex that I can find is an inflatable boxing glove, commonly called Sock'em Boppers. I found an off-brand pair at Dollar General for about $5.
It's simple to use and very rewarding! Simply blow it up, lube the entrance of the glove and your dick, insert, and hump away! It's a great hands-on, or hands-off method. I personally enjoy positioning it between me and the desk and humping it as I browse Solo Touch.
Clean up is relatively easy. I rinse it out with water, wipe out most of the water, and leave some dry toilet paper inside to prevent any unwanted mold. It's always good to clean it with soap, too.