Practical jokes??? Hmmm....
Well, short-sheeting a bed is a lost art, but definitely worth dusting off. It's a timeless classic.
One time we sewed my little brother's pajamas to his mattress while he was sleeping.
The BEST one I ever did may not be practical in your situation, but since you asked, I just have to share this one. This is a little long, but worth it

...
At work our department had an administrative assistant who was very nice, young, a little gullible, and had a great sense of humor. We joked around a lot in our department.
So here's what I did... I typed up a fake memo on company letterhead and printed enough copies to put on everyone's desks. I set it up so that when she left her desk to go on break at 10:00, everyone would put a copy of the memo on their desks and then leave before she came back. I put one on her desk and then I left too.
The memo said that the phone company was going to be
blowing out the phone lines at 10:25 so if you did not want to the dust to get all over your workspace, that you should put your phone in a bag and tape it shut.
We were across a large room in another cube area when she returned at 10:15 so she did not see us. We watched her sit down and read the memo. She immediately stood up to talk to someone else in the department about it, but there was nobody there. She sat back down and read it again, then stood up again hoping to find someone to ask about it.
It was getting close to 10:25 and she was obviously getting nervous.
It was all we could to to keep from laughing when she got out a Fed-X mailer and sealed her phone in it. Then...bless her heart... she bagged all of
our phones for us too!!
She went back and sat at her desk staring at her phone bag so, of course, we called her! Ha Ha!
The look on her face was priceless as she struggled with whether she should open the bag and answer the phone and risk getting dust blown all over her, or let the phone go unanswered.
At that point everyone lost it and began laughing and she spotted us!! We still laugh about it every time we see each other.
I can't take credit for the idea, it was done be a radio DJ in my home town. The best part about it was, when he announced it on the radio, people
at the phone company actually bagged their phones!
-Mike-