Princess Alphabet
I'd long considered a princess alphabet. I knew there weren't enough choices if you just looked at the princesses that most little girls think of when they play dress up, but by tapping into more pop culture icons for inspiration, I was was able to complete all 26 letters. This was even with giving myself the extra challenge of only adding actual princesses, not just characters that are princess-like.
What was especially fun about this was realizing that there are actually a wider range of princesses out there than when you first think of the term "princess."
Good luck guessing who is who. You'll need to consider movies, TV, comic books, video games and books. I'll give you one: Q is Qing Dynasty Princess.
EDIT: A new Princess Alphabet (with 100% more Elsa) now available.