Before doing anything else, you should take a look at your inventory, both your published work and the research you did to write it.
If you specialized in a particular subject, you may have enough information, or at least a good bit, to write a book on one aspect of your specialty. You may have been thinking about this for a while but never had the time to pursue it.
Opportunities for publishing go far beyond commercial publishers. While you could pursue the more traditional route, you can also look into self-publishing, either as a print-on-demand book or an ebook. Either of these will work well, if you already have a target audience.
You might also consider writing a blog. This shouldn’t be one in which you pour out your personal opinions, but a more professional one that appears online regularly and explores a particular subject.
Your blog could be based on your previous speciality or you could explore a subject is that is near and dear to your heart. The possibilities are endless.
And if you’re really ambitious you might try publishing a magazine—not one that will drain your financial resources but an online e-zine that mostly requires just time and energy. More and more people are reading about things online, whether through tablets like Kindles or Nooks or on their smartphones. An online magazine is just another extension of a blog.
You can pursue any of the above while still keeping your hand in commercial publishing, just not as much.
One thing you must do in order to enjoy your retirement years is to keep a flexible schedule, one that allows you to write at whatever level you wish but also allows you to pursue recreation and travel.