Playing tricks on people is fun, but make sure not to take it too far... You don't want to be mean, after all, just have a little fun. I find that getting thoroughly stoned people confused and having them play along with something ridiculous (convincing them that you're their teacher is a good example of this) is more fun than freaking them out, both for me and for them. Convincing stoned people that the cops/DEA/RA/parents are coming is not cool.