Can anyone suggest pranks to play on our church trip?

Question

Our church is going to Student Life At The Beach Sunday, and taking about 100 high schoolers with us. It's tradition for the seniors and college freshmen on the trip to prank the freshman and sophomores, while the Juniors remain in limbo. The problem is, we're having trouble thinking of something to do.

Here's the the situation: we're taking over part of a hotel floor, with four students in each room. We can't destroy anything we'd have to pay for later. Can you think of any pranks that would work? (We can most likely get into all of their rooms - our youth group leader is totally sympathetic to this.)

Answer 1

You can keep changing subtle things around their room thus making them doubt their sanity or think the place is haunted.

Answer 2

Greet you group leader crying "We like cookies" every morning