I am also retired, and working on my brain. I recently joined Lumosity. They had a special, $90 or so for two years. I have been doing their brain exercises for about a month. I am slow (after all, I'm from Tennessee), but flexibility is my strongest point brain-wise. After that, it's memory and problem solving.
I also do sudoku puzzles, crossword puzzles, play online trivia games and read as much as I can.
I have a recommendation for your French studies if you have some cash. Look on e-bay first. "French in Action" is a series of videos/DVD's with textbook and workbook that you can buy and learn the essentials of French. A telecourse was taught on PBS using these materials. The teacher is Pierre Capretz, a Yale professor (now probably retired). Yale allegedly has the best French program in the U.S. I follow along in the textbook as I watch the episodes. I love language, and if I had the means, I woud learn every language I could.