Usually, most people just dress up in typical Mardi Gras costume such as colorful masks or jester hats. And the colors for Mardi Gras are purple, green and gold. As to how to decorate your buggy, we use garland or tinsel and even some lights, and this to can be in the Mardi Gras colors.
Here are a couple pics from previous parades:
[img][img]http://i83.photobucket.com/albums/j302/wildthangbuggy/doodah02.jpg[/img][/img]
[img][img]http://i83.photobucket.com/albums/j302/wildthangbuggy/mardi58.jpg[/img][/img]
Hope this helps. You can also look on Jefferson Texas Mardi Gras website for ideas.
http://www.mardigrasupriver.com/index.htm