I have been having so much trouble finding a shower curtain. I wanted something that still let light into the bathroom, but had a little bit of detail to add some interest. I found some simple white ones at Ikea and you couldn't beat the price plus it wasn't vinyl, so non-toxic. This is good since I have two children that spend lengths of time in the bath. The shower curtain met the requirements but was so boring. So I decided to draw on it with an extra-large Sharpie marker. I mimicked the "faux bois" fabric that was at Ikea not too long ago, but with a little less detail.
Step 1: Tape down some craft or scrap paper ont a long table
Step 2: Tape down the first segment of the shower curtain
Step 3: Start drawing! Make sure to use a permanent marker. I stick with the Sharpie brand because I know that that stuff doesn't come out. 
Step 4: Test a small area to ensure that the marker doesn't run. If you are worried, you might want to put it in the dryer for a few minutes or iron it at a lower temperature.
I did a faux bois pattern which was simple because I didn't have to worry about the lines being perfect and could really improvise where I wanted a lot of pattern and a little pattern. I think you could just about do anything you feel like or even have your kids do one. The permanent marker might not be appropriate for them, but, non-toxic fabric paint would work just as well (just follow the instructions on the product). Have fun doing it!