The US government might have had good intentions with the "Cash for Clunkers" program, but that was an example of what not to invest in. Don't invest in something that losses value dramatically, like automobiles.
Invest in something that returns value. People need food, shelter, energy, transportation, etc. If a person could invest in order to have these things then that person would be investing in something of value.
An obvious example is of course energy. An investment in solar and/or wind power could really pay off. It depends on your own situation. Do you live somewhere with great sunshine and maybe even state and/or federal rebates on solar panels? Do you live somewhere with lots of wind and you have enough land to support a large wind mill?
I live in British Colombia, Canada, where electricity is (currently) cheap and solar panels cost twice what they go for in the States (why?). I am keeping an eye on what is going on in the industry, such as Lowes selling easy to install solar panels (in the States of course).
If solar panels last 25 years but pay for themselves in 5-7 years they could be a great investment. Something to consider when other investments are not looking so good.