Personalized everything is just fun. Albeit, having your name or initials on everything could get to be a bit much, but being able to decorate and design your own stuff is trés cool! I add my artwork to things like playing cards and drink coasters that you can personalize but, these sites let you do things that are arguably even more fun!
Marvel comics has a superhero creator on their Website that you can use to create your own superhero! As you can see below, I had some fun with that.
create your own anime avatar or create your own kawaii avatar, too.
Storybird is a site that provides writers with illustrations for their books. You can use it to create your own story. Need inspiration? Visit the site and peruse the illustrations. You may just find an artist whose work gets you going! If you have an aspiring author at home, then the British Council has a story maker for kids that's a lot of fun, too.
Inklewriter allows you to create your own interactive story. In addition to being fun, it's a great way for students to practice their computer and creative writing skills. The way their app sorts information would probably make it pretty useful as an online organizer - whether your publish your finished book on their site or not.
If you could create your own any thing, what would it be?