Kayak update: we’ve found several excellent kayak options, but the carrying rack situation is ridiculous.
We need a rack which will carry 2 boats atop a PT Cruiser, and the Thule and Yakima rack systems price out at nearly the price of a third boat! Any other, less expensive suggestions?
Those Yak racks can be handy for lots of other stuff, too, y’know. . . 
Depending on how wide the boats you want to carry are, you might be able to use foam blocks on your bare roof; you can get foam blocks shaped for kayaks at this link:
http://www.prolineracks.com/kayak.html
or this one:
http://www.xstreamline.com/kayak/pages/a_trans.html
some blocks also have cutouts so you can fit them on Yak or Thule racks, as well, if you decide to buy a rack system. And there are some rack systems less pricey than Thule and Yakima, if you look around. The Thule and Yakima racks seem to be the most elegant and versatile, though (as far as I know – I like my Yak rack, anyway).
–Aaron
“Quik-N-Easy” gutter mount brackets for making your own rack crossbars
are shown at the bottom of this Nantahala Outdoor Center web page:
http://www.noc.com/ss/racksys.htm
-Make a rack with these mounts, then add a vertical post for multi-kayak
carrying ability even on a skinny vehicle, leaning boats over on gunwales and securing them that way.