I should first point out that there's a difference between doing things to be funny and doing things that are outright mean. That said, here are some things that can be funny to do in the game.
1. Get a few friends and go into the different Gag shops in busy playgrounds in busy districts and then keep the gag shop clerks busy. Daisy Gardens, Donald's Dreamland, The Brrrgh and Donald's Dock are the best because most people end up in those playgrounds after a boss battle. This also works if there are toons there but the player is AFK for awhile. Not only enjoy the amusement but help keep a fellow toon from sleeping off. (For additional amusement, use the glitch to become invisible.)
2. At Cash HQ, use the flying glitch and position yourself over some tracks. Then have people teleport to you. If you time it just right, when the person appears, they will fall to the ground and be run over by a train. Even funnier if the same player keeps doing it and ends up sad, but only if they keep doing it when they already know what will happen, ie, is someone will go sad the first time it happens, it's not funny. (They won't lose their gags from it, so don't worry about them losing their mega gags.)
3. At Cash HQ, near the CFO doors, at the very top of the steps, the outer edge of the top step, there's a glitch. If someone is standing on that outer edge just right, they will fall through. Push people to that spot and watch them vanish to beneath the steps. Don't worry, they won't go sad or anything like that. I've done it and have actually had the people start their own little party down there once I had gotten enough people down there. Even seen a beanfest or two taking place there.
4. At Sell HQ, factory area, do the flying glitch and then push people back into the tunnel so they go to the first part of Sell HQ (where the pit and the VP doors are). I have a rule about this though. Only push the people that are between the two big blocks at the entrance to the tunnel. I only started doing it because I was tired of going through the tunnel to get to the factory area, only to be pushed back in by the enormous crowd of players there. When trying to get people to not crowd the area, I found out that most people KNOW it's happening, but they just don't care. So, I say, push them through the tunnel. Not only funny, but it encourages players to avoid that area, thus helping out others from being pushed back in.
5. When you're about to TP to a friend that's in another district, click the 'teleport' button multiple times to fill up their screen with messages. TP spam as it's called. Even more fun is to do it to the beat of "Shave and a hair cut, two bits."
6. In The Brrrgh, play 'King of the Wall' by keeping others off the walls. A lot of people play that anyway, so it shouldn't be hard to find participants.
7. Push people onto the trolley. It's harmless, keeps the player awake, protects them from being pushed onto a street where they could go sad and at the very least, they might earn a jellybean from it. It's a real gem if the person returns to the computer just as the trolley is starting to take them away.
8. When someone invites you to join a boarding group, if you are leaving the area, click 'Yes' after you are already leaving. You will appear on the list but then you'll have left, so they see someone joined by then left their group.
9. Use emotions in funny ways. For example, when someone is about to heal you using lipstick, use the 'Cringe' emotion. Someone using TNT, use the 'Banana Peel' emotion as it explodes. When jumping off of a high place, use the 'Belly Flop' emotion so you look like you're gonna land on your stomach. Use the 'Resistance Salute' to glide around instead of running. etc.
10. Use level one gags and high level cogs. This especially works if it's a squirting flower on a level twelve after someone uses organic TNT. Also using low gags on low cogs while in a boss battle, like a cupcake on a level 1 cog that isn't lured (or a squirting flower on a lured level 1 cog). So long as the cog will be destroyed, it's funny to use a level one gag in the process.
Anyone have things to do in the game that they think are funny? Anything that results in a player going sad or trying to make a player go sad doesn't count as being funny. So pushing an away toon into a building or a boss battle by themselves is NOT funny. Also trying to trick someone into doing a solo run when they can't handle it is not funny.