Popular Science & Google have pdf-d all of the past issues. It's lots of fun to look through them, and here's a couple of links for ideas:
a motorcycle towing a teardrop with a canoe on top!
http://books.google.com/books?id=eSYDAA ... &q&f=false
and a unique adjustable air suspension
http://books.google.com/books?id=ZyYDAA ... &q&f=false