First off, do not put glass cleaner or any cleaning products into the water! That could be downright dangerous!
Many companies have an office water cooler that just sits there waiting to be used for a really great prank.
The next time you change the water bottle, squeeze in a few drops of blue food coloring until it has that nice blue look and feel. Shake the bottle to mix the food coloring.
When finished, place a sign on the bottle that reads, "GLASS CLEANER." The water is still drinkable, but the visual impact is hard to ignore.