Hi, Mathew! You are correct - princesses seem to be just about everywhere. On the flip side of the coin, pirates are equally as popular. You can find pirate costumes and accessories throughout Walt Disney World, but especially at
The Pirates League at the Magic Kingdom. You don't have to go for the full pirate monty, although it is a lot of fun. They carry quite a few options to create your pirate costume in addition to the makeover packages they are known for.
I have also seen costumes for younger children, such as Mike and Sully from Monsters, Inc., and there is a huge variety of hats. You can pick up something like a Goofy hat, and just wear an orange t-shirt, jeans, and a vest to complete the look. White pants and a sailor shirt would make a great Donald Duck.
You might also find a racing team jump suit (think Cars), and increasing in popularity are the Star Wars accessories. It is likely that you'll be able to find the Jedi robes/cloaks, and there are several locations to find light sabers.
Have an amazing time on your upcoming trip! First trips can be absolutely magical.