For the wagon you probably have the most luck with a partial kammback.
My girlfriend drives the sedan kia rio and gets around 24/25 MPG but has gotten up to 28 and she does not hypermile even a little so 30 would be possible I guess, It just depends on how you drive.
Changing how you drive is probably the best "mod" you can do if you do it right.
a SGII would help a ton
Also This will help you look at how roof racks effect your MPG:
http://metrompg.com/posts/roof-racks.htm
And This one for smooth pizza pan wheel covers:
http://ecomodder.com/forum/showthrea...-6-a-4368.html