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).
Step 2: The Prank
Simply stated, fill each cup with water (about half-way is good) and place them close together on the floor of the area you'd like to obstruct. The closer together, the better. You should not be able to step between the cups (or at least not easily). Your prankee should find the set up and look upon it in awe, then quickly switch over to annoyance when they realize they have hundreds of tiny cups of water to dispose of to get to their bedroom/office/bathroom etc.
Step 3: Tips
Though simple, this prank can take a while to set up, so make sure you have the time to pull it off. If you're caught mid set-up, you'll probably be stuck dumping out the water and disposing of the cups while your prankee glares.
Friends are helpful and so is a nearby sink. An assembly line makes this move much faster.
Watch out for doors that open into your prank space and anything else that could knock over the cups. The goal isn't for your prankee to open their bedroom door and take out two feet of cups. You'd rather they to try to walk through the cups and THEN knock them over. Or knock them over while cleaning them up.
Step 4: Tricks
Get creative! Floors aren't the only surfaces that can be covered, anything that's relatively flat will do. Cup someone's desk, car, kitchen table/counters, or closet floor. Remember, the goal is extreme inconvenience!
Make designs or elaborate figures. Add a prize to the mess (say in the middle of 6 feet of cups) to offset the prank if you want to be nice.
Step 5: Mean Tricks
If you want to be mean (and colorful) fill the cups with colored water or Kool-aid (or dye if you're really bad). You could also fill the cups with something your prankee wouldn't want to dump down the sink (like good booze). Of course this would increase the cost of the prank, but it might be worth it.
Prankee have a cat or dog? Toss an animal into the mix and cups will tip.
If you can time it, consider punching a pin hole in the bottom of some of the cups. Leave them be or place a square of paper towel underneath to soak up some of the leakage. In this prank, nothing says "panic" like leaking cups.
Connect a few cups with a piece of thread (taped to the bottom of the cups). Your prankee will pick up one cup and empty a handful of them all over the floor.