I found this link for a car seat cover on pinterest and figured it'd be pretty easy to make. When Simon was born, we didn't have a car seat cover, and just used a blanket, which always blew off with a wind gust. This is nice because it actually buttons on, and seemed like a great way to keep the little one warm in the cooler temps we are getting now. I followed the tutorial pretty much the whole way through, and liked the outcome. The hardest part was the button holes.
The pics are terrible with bad lighting and from my ipod, but you'll get the idea.