I accept to be LegoDnD's roommate and start to create loud annoying sounds that the whole world starts to track the hidden hobbit hole down. I leave an exposed exit/entry hole for the next user to find that has no security and is easily accessible to anyone.

I go into the hole, set a bomb (only a conventional bomb, not a nuclear one or one with super-astronomical strength), and leave before it can explode.

Once it explodes, I go to the remains, and leave some Resurrection Powder for the next user.

With my home compromised and sense of kinship betrayed, I gather the powder and secret it away in a high-security vault in my basement.

I leave at my new front door a powerful laser-turret under an invisible tarp.

I take the invisible tarp, ignoring the turret, and fashion a nice outfit out of it.

I leave the invisible clothes to the next user.

I leave you the key to my apartment, as the invisible clothes just got me thrown into jail for indecent exposure.

My small apartment is filled to the brim with all kinds of art supplies and martial arts equipment. I hope you'll pick the right tool to break me out of jail. Assuming you only have two hands, you can only pick two items, for example a No.7 Kolinsky sable brush and a steel longsword, or a can of paint thinner and a knightly poleaxe - it's all up to you now.

I guess "ignore" is your way of saying "disable" then.

The ax-head snaps off as I pull the long handle from the engine of your car, but I take that long pole and paint thinner to Silent Hill and leave you high and dry in jail. What creatures I find there, I leave to the next person to decide.

I discover a disintegrator beast in the area, and take advantage of its ability to disintegrate the walls of the jail, allowing me to escape.

Then, I decide I'm in the mood for a little number theory, and leave, for the next user, two distinct even primes. (One of them is 2, but what's the other one?)

I googled "second smallest even prime number" and was told there is no such number. I fill up a swimming pool with
pistachio-flavored pudding and leave it to the next user with the hope that they invite some swanky guests and throw a party with the pudding as the star attraction.

I invite the neighborhood cats via catnip on a pull-string that leads to the pool. Whatever happens next, I leave to the next person.

Seems there are no cats in the neighborhood, so none show up.

I leave a mathematical proof for the next user. The proof begins with "Let p be an odd prime"

I hand the proof to my boss and get promoted to CEO for "I" have founded a new discovery.

I leave the forum updated so the next user and make a new post on this dead zone area

I take the updated forum and find a flaw in it that allows me to crash the forum, making it unusable for everyone.

I then leave it to the next user, where "it" is the subject of the sentence "It's raining.".

I find myself in the opening scene of The Happening where the suicide wind has drifted over a construction site and it's raining men. I leave a radio next to a megaphone to blast the song It's Raining Men. If this bothers the next user, they'll need to turn off or destroy the radio, at risk of getting crushed under men who were most likely exchanging an alien's idea of human small-talk just a minute prior.

I get the bright idea of letting the radio be crushed under the weight of one or more falling men, and so I leg it as the chorus is cut short at 'absolutely soaking wet'. I am leaving it to the next poster to make sense of the notion 'alien small talk'.

I hear those aliens talking about being small, and I almost step on them by accident. Oops!

At least, it's only "almost", but I did, unfortunately, scare them away.

I leave a Pathfinder (or D&D 3.5, if you prefer) character sheet, showing a character whose classes are Cleric/Rogue/Eldtrich Knight.