I don't know much about watercolor, but to make it more fun & interesting, why don't you go with your idea of masking shapes or stripes, and use them to make greeting cards? You could use that as a background and then either use pens / ink to write on top of it, or cut another layer to glue on top for some text or other shapes. Valentine's Day is coming up so you could do some cute cards for that, or just regular notecards, birthday, etc.
It's great that you're sharing your talents with your friend. I hope the two of you have fun!