well, there are obviously many ways to skin this cat
you could make it rain for forty days and forty nights. (sort of time consuming, really)
stop up all the sinks, and turn the taps on full blast. (sort of apparent to anyone who happens to walk by, so not too advisable)
pull a fire alarm (REALLY not a good idea, unless you are into police investigations..)
and then there's the method I used back in the second grade:
(wet toilet paper is quite amazingly watertight, I have found...) For some reason, when I used to do this at school, it didn't occur to me that I was actually causing the bathroom to flood when I did this; you're kind of dumb like that, in second grade... anyway, I never did get caught, although the principle came around to all the classrooms and said something like, "if you know who's doing this, please tell them to stop." (Mr. Upper, if you are reading this, "sorry!")
- lift the lid on the tank of the toilet
- wet a piece of toilet paper (double ply, preferably), and place it over the overflow pipe
- unscrew the float from the rod that controls the flow valve. (or, for a delayed effect, screw the float more tightly, so that the flow slows, but doesn't shut off)
- put the lid back on the tank of the toilet
- get out!
confession is good for the soul....