How can senior citizens effectively contribute to sustainability?

With baby boomers in North America and baby boomer equivalents in the rest of world right now in their post-retirement phase of life, the world has tens of millions of senior citizens with significant skills, and also with time on their hands to utiilize the skills productively. Most important, many of them are keen to do their bit for the planet. How best can we utilize this amazing asset in the form of highly capable and motivated senior citizens to make the planet more sustainable?