Seniors today may well enjoy rounds of golf and water aerobics when they retire. But chances are, they fit it in when they’re off the clock. Whether motivated by mental stimulation or cold, hard cash, millions of Americans age 65 and older are keeping one foot in the workforce long after they leave their full-time careers.

Among current seniors, more than 16 percent were still in the labor force in 2010, up from 12 percent in 1990, according to the U.S. Census Bureau. A poll last year by the Associated Press-NORC Center for Public Affairs Research also found 82 percent of workers age 50 and older believe it is somewhat likely they will work during retirement. Often, that means consulting gigs, part-time jobs close to home or entrepreneurial pursuits that convert personal passions — such as carpentry, tutoring or public speaking — into income. Read More: Women face retirement-saving challenges “Lots of older Americans are taking advantage of self-employment,” said Kevin Cahill, research economist for the Sloan Center on Aging & Work at Boston College. “We’re in a new era of retirement, and we’re not going back.” He added that “most people assume that seniors keep working due to financial necessity, and some do, but the majority do it to keep active and stay alert.” There’s no one right job for all retirees, of course, but there are industries and opportunities that are far more likely to meet their needs, be it supplemental income, social outlet or mental challenge.

Those who seek to maximize their paychecks, for example, are best served by taking their education and experience on the road, said Kerry Hannon, author of “Great Jobs for Everyone 50+: Finding Work That Keeps You Happy and Healthy … and Pays the Bills.” Touch base with your ex-employer first to find out whether you can assist with a new or ongoing project on a consulting basis, said Hannon. But by no means should you limit yourself to your former field. Look to other industries that may also have a need for your skill set. “You’re not reinventing yourself,” Hannon said. “You’re redeploying your skills.” Retired accountants, for example, could get certified as a chartered auditor or financial advisor, while bankers and office managers can advise on process improvement and cost control. Marketing and public relations professionals, meanwhile, can assist companies with pricing, long-term growth strategies and brand image. Opportunities abound.