When my twin boys turned two, I put my all into their birthday party. I selected a bug theme after seeing the cute ladybug invitations idea in (I think) a Family Fun Birthday Ideas booklet (since then, I've seen this idea ALL over the place).
I designed the invitation on the computer using Freehand, printed them out and cut them by hand. The brads are the GINORMOUS ones that you can buy from any store for kids' projects (I really love the newer mini-brads, much less imposing).
Anyway, I got a lot of great reaction to these invitations after I sent them out. Getting that kind of feedback was a bit addictive. I worked even harder on the following year's invitations, which were, basically, my spinning fish cards, which were among the first things I listed on etsy.
Last year, I designed the spinning circus invitations for the boys' ninth birthdays. People LOVE these -- I get a ton of feedback about these. The fun thing about these is that the text (which is all customizable) scrolls through the white banner while fun, circusey images appear in the doorway of the tent below. When the birthday child's name is mentioned, their face shows up. I get amazing google search engine positioning on this item.
In the spring, I made invitations for a home brew party that we threw. I was so pleased with the invitations I designed (which were a variant of my Beer Buddy Cards) that I sent out more than double the number I really should have (resulting in a huge, but fun, crush).
Since then, I found an outlet on etsy and have added a few other things to the line-up besides these, but these three items are still among my very favorites and are perennial best sellers.