Hi,
Bubblewrap on windows has a less than one heating season payback in our climate. Actually, it has a zero days payback if you get the bubble wrap free
The $22 sunspace in the Feb 2005 issue of ESSN should have a less than 1 year payback:
Rebel Wolf Online -- ESSN Archives
If you have not seen these issues of ESSN (all free), there is some nice stuff in them.
Painting the south side of your above ground pool should have a really short payback:
A VERY SIMPLE pool heating idea
This sliding glass door insulation project has a half year payback if I did the numbers right:
Insulating a Sliding Glass Door
Gary