Cari,
I've done most of those. My all time favorite is sticking a little piece of tape over the part of the phone you talk into. It will go undetected for days meanwhile everyone they talk to will be asking them to speak up and repeat themselves. They'll turn the volume all the way up thinking that might be the problem and usually don't have the common sense to even look at the handset.
Another one is just a simple post it note taped on the back of the chair. Very juvenile I know, but funny.
This girl I know very well, we're friends. I have the password to her computer. I go into her screen saver and change it marquie "Rachel is Queen", then change the password. I do this when she's in but away from her desk.
Our phones have a screen which displays 6 speed dial names. I switch the names to say stuff like
Rachel
is
hot
I
am
gay
Just a few of my favs.