E.B.
We did a monkey theme for our kiddos/ guest bath. It turned out really fun! The original stuff I bought was so generic and since I have a boy and a girl I wanted to make it a little more fun for both. So I hot glued bows on every other monkey on the shower curtain, making it boys and girl monkeys! (Target set, I love it!) and had each of the kids names put on the decorative towels. I also got some fun art work of monkey's in swimsuits, one boy monkey and one girl monkey for hanging up on the wall and a couple of cute monkey hooks for their towels to hang on the back of the door.
A friend of mine went with a prince and princess frog theme and that turned out great as well using green and then accenting with some cute frog decorative items and hand towels etc.
If you're looking to do something less childlike you can always have fun with adding stripes or polka dots in art work or towels to add some other colors and a fun theme without actually making it a "kids" bathroom. Just depends on your preference!
Have fun! I love decorating! :-)