This is a good one that is on about the same level as the prank they pulled on you. No lasting damage real funny and pretty clever imo. You do need access to the inside of the car though.
Take a small can of tuna fish and make just one small hole in the lid. Drain most of the water (no need to spill it on the guys car unless you really want to step it up a level). Find a place to hide the can. Sometimes you can get it into an upholstery fold under a seat. The place should be really hard to find, because this prank develops over time. The tuna wont start to smell for several days, unless its really hot. The smell will get worse and worse as the days go by, but its not going to linger in the car too long after he finds the can. He will go nuts trying to find the source of the smell, eventually he will find it and realize how much better your prank was!
Another funny one is to place a fat slice of cheese on the radiator or somewhere on top of the engine. As he drives off and the car warms up a funny odor and eventually some smoke will develop. Again harmless, but more immediate.
Go through magazines that have cologne and perfume sample sheets and slip a bunch of them into the vent slats on the inside of the car. Every time he fires up the vents or AC, he will be bombarded by fruity smells. This one lasts for awhile unless he figures it out and opens the hvac system... can be a hassle.
This is a real winner, and is immediate. Go to a hardware store and find axle grease... it has to be a real heavy dark grease... I have found that axle grease is almost impossible to beat for this prank due to its sticking characteristics. Carefully (with gloves!!!!!) smear a bunch of grease on the underside of his door handle. The beauty of the axle grease is how difficult it will be to remove from his hands, and he wont want to get in and drive the car right away. You can also use silver nitrate. This will turn his fingers and black/bluish color for a couple of weeks. No amount of washing will take it away. This happened to my dad a few years back when he was parked outside of costco. I laughed!
I think all of these pranks are relatively safe and they wont damage his car permanently. Don't do anything that could interfere with the operation of his car or cause damage. Sugar in the gas tank, blocking his fuel line... are you kidding me? These are all things that could kill the guy or at best cost him a bunch of money/time to repair. They are grounds for a lawsuit or at a minimum will cost you your job. Think carefully about what he did to you, and do something to him on the same level. Joke about it afterwards, make sure the guy doesn't take anything personally and want to "up the ante" to an extreme level.