For every birthday occasion for my kids, I try to do some craft of some sort - either involving the guests to make it, or having the guest add message/pictures to something I made.
Here is a train that I created with My Community Cricut cartridge. Inside each of the trains, I put a picture of each guest. I also added some of my son's favorite characters into the trains as well (Mickey Mouse, Thomas, Vancouver 2010 mascots...) I cut out the shapes big enough for all the guests to write a birthday message on the back of their train. Of course, the train driver was the birthday boy.
I attached all the trains together to look like this:
This not only makes a great birthday party decor, but a great memory keepsake!
(The lettering was done with SCAL)