This usually works out better if the other person has a desktop PC, because you can hide the wireless mouse or keyboard receiver behind their PC, and then every so often type an extra letter, or move the mouse slightly. If you do it right, you can have some fun with this one for a long time. The great thing is that this should work for almost anybody, including people that keep their computer locked all the time.
If they do have a laptop and a wireless mouse, you can simply replace their wireless mouse with an identical-looking one, since it won’t work for their PC. They’ll assume their mouse batteries are dead, and you can move the mouse pointer around the screen with their mouse. The prank won’t last as long, but it will be every bit as fun.