A Good Bicycle
I don’t mean to sound like a sporting goods salesperson here, but have you ever worn a really good pair of running shoes, after shuffling about in ratty sneakers made before the invention of arch support? Taken a bike ride on a really good bike, after years of tooling around on a rusty old ten-speed? There is a lot of new technology out there that actually makes certain sports easier and more fun. See also: skis, surfboards, pogo sticks, etc. And what could be a better use of your money than investing in something that makes exercising more comfortable and fun and therefore likely to happen more often?