Everyone loves a good practical joke! As adults, we can still find ways to bring a little fun into our mundane lives. You can always go with an old standard, like running around your neighborhood at night ringing all the doorbells, or dressing stray animals up in retarded looking party hats. But if you're really looking for a laugh, try taking it to another level, and do something they'll never see coming!
A real friend is the best target for a harmless gag! A whoopee cushion is little played out, so try wiping your ass on the back of their bathroom curtain instead (don't worry, the sun will bleach it out)! Then head to the bedroom and slather all their clean pant crotches with egg whites. Maybe find a little time in your week to meet their fiancee for lunch, and say things that gives them doubts about the relationship!
As you've noticed, I'm a tough customer, which means I'm always running into problems at various businesses. If I don't get my way I can sometimes subdue my rage by pouring it into a well planned prank! Like once I managed to sneak some buck lure into the coat room of a fancy restaurant so I could douse everything in raw, musky goodness. I've also been known to plant drugs by the register and report them anonymously to the cops.
Sometimes I like to play tricks on total strangers. You can find a car with an open moon roof and pour a can of potato soup into the driver's seat. Consider prank calling a nice old person, even when you're sure they don't deserve it. Fill a balloon with grenadine and toss it at some bridesmaids. Or maybe you could break into someone's house at night and shoot them in the extremities with their own guns. And nothing says "fun" like loudly mocking homely children at the mall!
Like most parents, my favorite jokes are the ones I play on my kids! Children are so naive that they'll fall for anything, so keep it simple. At night I'll pour them a nice hot bath, but then fart in the water before I let them get in. Or I'll serve them up a country ham for dinner, but tell them that it's a smoker's lung. And when they're especially naughty I'll threaten to flush their dead grandfather's ashes down the toilet! They fall for that one every time!