Do you have empty water bottles piling up, just waiting to be used?

Well, I've got a solution for you!
(I apologize for the salesman-like opening, I just wanted to start it off interesting, yah know?)

First, take all your empty water bottles and fill them with water.

Then take some food coloring (you might want to make sure you have that before you fill your empty water bottles) and squeeze a desired number of drops of food coloring into each bottle.

Then line up the bottles where either a) the sun can hit them and light them up or b) they can be lit from behind by some form of lighting.

What follows is a description of what I've done with this process.
As a college student, and living in an arid part of the world, I drink lots of water. And flats of water bottles are cheap. I have drained 5 to 6 dozen bottles of water in the last 3 months. This is why I decided to experiment. I also thought it'd be cool.

Most of the bottles are lined up in one of the windows in my dorm lounge. The window is high enough up that only a really tall person can reach it, which my roommate happens to be. The only way they are coming down is if someone goes to drastic measures (i.e. a ladder, long stick, or getting someone else equally tall to snitch the bottles) to get to them. This lounge feels really bare, so I was trying to effect the lounge by messing with lighting color. Each bottle has one drop of red food coloring in it. This makes them kind of pinkish, because one drop is really quite weak in that amount of water. The effect, however, is pretty cool. Anytime during the daytime you can go in there, look up through the bottles, and see this sort of shiny pink illumination.

I also have some bottles lying on top of one of the lights in my dorm room. They are about 1/4 full, so the weight doesn't kill the light; and on their sides, so I get more area coverage by less bottles. These ones are a mix of colors. Some have only one drop of blue food coloring in them, some have a lot of drops of blue food coloring in them (they are almost purple), others were attempts at making purple by mixing one drop of blue and one drop of red in the same bottle, there's also one bottle with a drop of red in it to add contrast to the whole set up. This makes really good, really cheap mood lighting. And it looks pretty damn cool to boot.

The last place I have my bottles at is in my front window, which looks out onto the passageway where everyone in the dorm has to walk if they want to get to the elevator. These bottles were more to experiment with color drop combinations than for any particular effect. I've got several of the previously mentioned 'pinkish' bottles in my window along with a bunch of others. Theres an orange bottle (one drop yellow, one red), a weak purple bottle (probably 1 red, 2 blue), a couple of dark red bottles (lots of red drops), two dark blue bottles (lots of blue drops, one is slightly darker due to more drops), one really really dark, almost purple bottle (possibly 1 or 2 red and then many drops of blue), and two bottles that look like a dark crimson (these were a lot of red drops with a lot of blue drops, also my favorite two in the group, though they look black when not lit by some kind of light). I usually keep these bottle back lit by a lamp on my desk. Anyone walking outside can see the colors a lot clearer than I can, and they look very nice. When the sun gets low enough, it shines through my window through these bottles. The colors that play across my room from this light shining in, are undescribably amazing.

The bottles in my window are also good conversation starters.

As a result of my experiment I am now almost out of blue food coloring. Oh well. I've also used up all the available space I have for my bottle. My roommate suggested putting them in the lounge of the next dorm over, which has exactly the same layout as ours. As soon as I get enough empty bottles again, I just might do that. Oh yeah, I'm also out of empy bottles. This experiment was highly effective. If you choose to do something like this, I highly recommend that you just play with it. Thats the best way.