Just to play devil's advocate, if you are coming on a forum to ask advice (from scratch) about how to invest your parents retirement savings, are you necessarily the best person to be handling their money?
My suggestions would be to hit the library and get a few books on investing, basic economics, debt (as in corporate and gov't debt not personal debt) and financial planning. Once you have a couple ideas of where to invest maybe came back to have discussion on the pros and cons of the various ideas. This is your parents' retirement savings and can't be replaced if a poorly researched decision is made...
Good luck.