My dad once brought home a coworkers desk drawer over the weekend and made lime jello.

On Monday morning he quietly reignited his old rumor that she kept jello in her desk.

In annoyed fury she flung open the drawer to reveal it completely filled with jello, one unpeeled banana in the middle.

Buy large bags of Christmas themed M&M's, Reese's Pieces, and Skittles. Mix them together in a single bowl. Don't tell anyone.

Wrapping up their office?

Stapler in jello is so stupid anymore that it's circled back around to being hilarious

Wrap everything in Christmas wrapping paper

Like, she doesn't have a office with a door?

One year, we got a tank of compressed air and a bunch of balloons and stayed after work and literally filled his office with balloons. Floor to almost ceiling. Not only was it hilarious, but popping the balloons the next day was a lot of fun, and clean-up wasn't too hard. You absolutely need compressed air for this, and even then it took an hour or so to do.

These days, I guess it'd be seen as environmentally wasteful; balloons aren't recyclable, I think. Plus, it only works if she has dedicated office with a door. If it's still an "office" but shared with another person, you could still get their permission; maybe they'd agree to hotel in a conference room until the balloons are popped.

Buy three non-venomous snakes from an exotic pet store. Paint their scales with labels #1, #3, and #4. Hide them in one of her desk drawers. After she overcomes the initial shock, she'll spend the rest of the day wondering where snake #2 disappeared to.

Wrap her keyboard, mouse, etc. up in gift wrap.

If you can access her computer, you could mess with the autocorrect settings and replace common phrases with Christmas carols. Like, change her name to 'Santa Claus'. Or change 'Kind regards' (or whatever she signs off emails with) to 'Jingle bells', etc.

Put up Halloween decorations and claim you got the date wrong.

Opaque tape on bottom of her mouse, where the laser is.

Without detaching the pepsis from the band that allows them to be sold six per pack, empty them and fill them with coke, then say you bought a six pack of pepsi. Or do the reverse.