Tiny Plastic Hands
Okay, so it isn't a true exchange as you won't have to give up two of your favorite appendages permanently. Instead you hold the handles of these gag items, discreetly hide your real hands in your sleeves, and let the fun begin. Just imagine extending these out to accept your order from the drive thru window, or using them to hand over payment for a set of movie tickets. Bring a little joy into your life, and the lives of others, by sharing a good laugh with friends and strangers alike.
If you really want to go for the gusto, use them to shake hands at your next job interview (be aware this may negatively impact your chances of actually getting the job) or when meeting your soon to be father-in-law for the first time (again, warning, this can definitely reflect negatively on you and you may pay for this the rest of your life).
Up the risk to up the reward! Maybe let someone grab it for a handshake, then release the tiny hand from your grasp and watch the target of the joke try to deal with pulling your unnatural hand off. But, do remember to use with caution; with tiny plastic hands comes great responsibility, and a naïve sense of optimism that people share your sense of humor.
These plastic monstrosities are a whooping three inches long making for an obvious difference when used on most adults. The craftsmanship means they will appear as actual hands at a glance, just much smaller. Working under the same uncanny resemblance principles involved in zombie movies, many people become unnerved by something that looks just slightly wrong and catches them by surprise.
At this time, they appear to only be available in one shade of skin color, which I can only label as the standard Caucasian beige used in many children's dolls, but don't let that hold you back. Imagine going to shake hands with someone and not only are they tiny, but they don't match the rest of your skin tone either. That may result in more than just a double take by the poor soul subjected to this prank.
It should also be noted that the fingernails appear to be darker and slightly orange. This gives the appearance of some strange nail polish choice, though it could likely be painted over to a more trendy nail polish shade or a classic French tip manicure. However, I don't know if the gag is worth that kind of effort.