Unscrew the shower head and fill it and the nozzle with Jolly Ranchers. She'll wonder why she's all sticky when she's drying off.
Or use bullion cubes instead.
Remove the ball from her mouse if possible. Lame but annoying.
When she returns home and stops by to let you know she's back, imagine how surprised she'd be if you answered the door wearing some of her underwear. This might be crossing a line.
A fine coating of flour under the sheets of her bed will cause issues if she's a sweaty sleeper. Again with the line though.
Conspicuously leave a kitchen utensil of some sort in her toilet. Add Mt. Dew/beer/yellow food coloring/pee to the water for effect if desired.
Unscrew and remove the plastic screws that hold her toilet seat in place. Leave the seat where it belongs. Hopefully she won't end up with a concussion and a spatula up her ass when it goes sliding off the first time she sits down.
Go to the pet store and purchase a small army of crickets. Set them free in her room. I think they're about a dime apiece. That's $10 well spent.
Purchase a package of hot dogs/brats/sausage/whatever. If you have an eight pack, leave six or seven of them laying around the house in obvious places. Leave the wrapper on top in her garbage can. Eat or dispose of the other(s) elsewhere.
Take her TV remote.
Altercall was close to my personal ace in the hole with his alarm suggestions. While I don't think you'll be able to take delivery of it in time to place it this weekend,
THIS (especially if you splurge for a few of them) is your Little Boy and Fat Man all rolled into one. Trust me. It is worth every penny. Your first response should be to feign ignorance. A week later tell her maybe you might know something about it. A week after that, give her the empty box (if you hide more than one, don't give her all of the boxes). A week after that and she'll do anything you want as long as you tell her where you hid it/them.
Whatever you decide on, I totally agree that you should make a copy of her key while you have the opportunity.