Quick and (Very) Dirty Marble Painting

I'll never find it to link to it again, but a few weeks back I watched a video of a man "painting" by rolling marbles and spinning tops across wet paint. I thought "Hey, we could do that... I mean, except for the really cool part where he actually makes the top draw a recognizable picture. That's not happening, but abstract? All the way!"

So we tried it. Our results are not as cool as his, but since we're not linking to the video, it's not like you'll really know, right?!
This is a project that can appeal to all ages and takes nearly no skill (aside from not popping the marbles out of your container).

Materials

1. Acrylic paint in 2-3 (or more) colors
2. A rimmed art tray, like one of these, or (considerably cheaper) a foil tray from the Dollar Tree meant for lasagne or casseroles. Just make sure it's at least 8.5 x 11 inches and fairly deep, or do the project Outside!
3. At least one ordinary glass marble. Or maybe two or three! 
4. Paper cups or foil plus popsicle sticks or something for mixing your paint
5. Whatever protective clothing and surface coverings you feel are justified
6. Solid colored paper or cardstock

Instructions

At its most basic, you're adding a few splashes of paint to your tray, dropping in a marble or two, and then rolling them around.
And feel free to start out by doing just exactly that.
But here are a few tips I found after messing around for 20 minutes that gave me the results I like the best:

1) Thick, tacky acrylic does not work very well. Add a few drops of water to your paint in a mixing cup to thin it considerably.
2) You can apply the paint directly on the paper, but this made blobs and globs I didn't like much. I ended up applying the thinned paint onto the tray, up at the top above the paper.
3) I think using one color of paint at a time is best. If you use a couple of primaries at the same time you could get some interesting mixes, but I think most likely you'll end up with mud.

Now the next question is: what to do with the finished product?
I think the idea that seems most practical to me is to add some adhesive letter stickers or shapes to the blank paper before starting as masks, then carefully peeling them off.
Other ideas are to make origami boxes or other creations out of your dried project. Or cut into pieces and use to make mosaics. The sky's the limit!