This is a classic prank that's easy and annoying as hell. It was pulled on me a few years ago and I was pissed, but impressed.
In the following steps I will explain how to put together a water-based obstacle that will annoy anyone and everyone and add a few tips for making it really evil.
Step 1: Materials: Cups and Water
I recommend little Dixie cups because I find they make this most annoying, but any plastic cup will do, large or small. You also need a water source (sink, bucket, bottled water, whatever). Depending on the size of the area you're going to prank, you may need a lot of cups and a lot of water (i.e. you might have to spend some money).