Talk about an instant gratification project! This one is super easy and super fast. It is by far the easiest marbling method that I have tried so far.
a plastic bin that you can get paint on (I used a $2 plastic dish bin from Big Lots)
some cardstock or watercolor paper (any slightly heavier weight paper that isn't glossy should work)
spray paint (I had the best luck with dark colors and metallics)
Fill your bin half full with water. Spray the surface with spray paint. Drop the paper on the surface. Turn it over. If the paper didn't catch as much paint as you wanted, dip it in again. If you want a more "stirred" pattern, shake the bin a little to mix up the water before you put your paper in.
Here's what the process looks like with some unstirred black and silver spray paint (click the photo to make it larger):
Some things I learned from this project: WEAR GLOVES! I got spray paint all over my fingers from dipping and removing the paper. It took days to get all the paint out from under my nails. Work fairly quickly. Spray paint dries fast and you may end up with some odd results if you're messing around trying to take pictures between when you sprayed the paint and when you put your paper in. Other than that, it's super easy and really fun. The next time I do this project I'll make sure I have more dark spray paint available and I'll probably try out stirring/combing the paint with a bamboo skewer--although you get pretty cool results with just jiggling the water bin a bit.
I can't wait to use my marbled paper to make some cards and gift tags. In the meantime, I took the one piece of paper that had marbling only in weird spots (I was using this piece of paper to clean up the excess paint in the bin before I started with some fresh colors, so the leftover paint didn't cover the whole piece of paper), and I cut some strips out to make some cool bookmarks. I ran the paper through my laminator to make them durable and glossy.