Last year, when my girls were a year old and just learning to walk, I made them penguin costumes.
It was super funny because they waddled like penguins anyway! Here's a link with a bunch of pictures, and even some video! (Oops- link below)
This year? I making them TWO sets of costumes! For the first one, my family is going as a Star Wars experience. Yeah, we're nerds. I'm going to be Princess Leia (white robe, not slave Leia), I'm making my husband a Chewbacca (or, if I run out of time, Han Solo) costume, and the girls? An Ewok and a Jawa. That's just for the parties with our friends.
The other costumes are for the child-oriented activities. I'm making them kangaroos. First of all, they won't need to carry a bag- they can just put their candy in their pouches! But most importantly, they LOVE to hop. So, I'll have two hopping kangaroos. Like last year I had two waddling penguins. :)