A speech topic that would be appropriate for a group of retirees would be, to me, to explain that once retired from one's main employer life and one's aspirations and drive do not end there. For example, it is still possible to earn money but less strenuously and rigid than punching a clock by, for example doing freelance work on the internet. Also, now that there is more time, one can pursue his/her real interests of say, writing a book, becoming an artist or supporting progressive politics, for example.