E.T.
For my son's third birthday, we contacted a local fire department. They drove a truck to our house and let the kids climb on it, explore, etc. They loved it and the pictures are fantastic! We bought cheap fire hats for the kids. I made a firetruck cake (there's a cool design for one at Familyfun.com). Look at that website: they have a lot of great ideas for themed birthday parties. My son's friend had a similar party, but they drove the truck to a local park where the kids had a playground etc. to play on.
Funny, I had a similar situation. My younger son was due on my older son's birthday! I mourned his only-childness, but I have to say that seeing them together, I realize how great it is (and has been) for them to have each other. My younger (who had the firetruck come) is now 11 and my older is 14. He took the bus from his school to come to his brother's school fair! I love how they support each other!
E