For some bizarre reason, most people think pranks have to be mean spirited. This is hard to explain when you know the tradition of pranks dates back to the Celts, All Hallow Even', and what we know know as Halloween. The original idea was that there were spirits haunting the countryside and if you wanted those spirits to leave you alone you had to put scary things like jack o lanterns outside your door and if you had to venture out dress as a ghost yourself. These spirits were known for having a rambunctious nature and would pull pranks on people such as moving the stuff on the porch, knocking on doors and leaving before you could open it, and so on. Pranks continued to be funny jokes on people for a long time, culminating with the Halloween classic of toilet-papering someone's property if they did not give you a treat (hence trick or treat). More adult jokes or pranks came the forefront later, such as dog droppings in a brown paper bag set on fire on your doorstep.
Halloween is perhaps Christmas for adults. The one time of year when grownups get to act silly, even at work, and get away with it. Some estimates suggest we spend more per adult on Halloween than on Christmas itself. Although pranks are often mainly associated with April Fool's or April Fools Day, pranks are also the funny stuff of Halloween. No matter the season, there are some pretty neat and harmless pranks out there. The Whoopee cushion is a classic, and so is the dribble glass, various noisemakers hidden in inconspicuous places, and hand-buzzers (always annoying and usually used by big hairy guys with gold medallions). Pranks can also be anonymous, such as placing an insulting bumper sticker on the bumper of the idiot you work with or the schmuck who stole your parking spot at the shopping center. Investing in a prank, a good clean adult joke, funny stuff as a way of getting even is probably cheaper than therapy and a lot more immediately rewarding.
You do not have to spend a lot of money on pranks. If you really want to get someone, use the internet to put this person's telephone number on all telemarketing lists. Use this person's email address each and everytime you visit a site that you know will spam you. There is nothing easier in this day and age than to pull a good prank such as getting someone on every possible and imaginable mailing list; religious groups are a good source of heavy mailing lists as are various so-called charities (the fake or highly questionable ones, not the real ones like the Red Cross or Greepeace). If your target is a vegetarian pull a prank on them by getting their address on the lists of various meatpacking companies and meat product companies. (Spam them with Spam!). Adult jokes can be funny stuff as long as the cost is lower than the desired consequence and that there is no lawsuit at the other end.
Pranks are also a good way of relieving tension. They should be good, clean, funny adult fun. Jokes that work without really hurting. Invest a bit in stuff you can stick or mail, fake parking violations, fake products, etc. There are many places that specialize in prank ware and that would be happy to help you pull a good funny prank on someone.