If it is this year (2009) you could defiently do something with the Celebrate theme that the Disney parks currently have going on!! I would also check out the DISign boards because there are some really creative people over there! Good Luck!!
it may be out of the price range but
get a few mickey head balloons and have the guests pop them. inside a card would have all the details of the party and stuff. I guess if you wanted to put a small piece of paper like a fastpass ticket in the actual invitation so they would be able to get in. You know what im getting at?
Good Luck!