There are abundant opportunities to keep your body and mind busy, and some of them will make you healthy, help you get into college or grad school, earn you some money, become skills you use all your life, or open doors to activities you will enjoy for years to come.
Take up a solo sport. Become a distance walker, runner, or weight lifter. Go hiking or cycling, take up archery or yoga.
Become an expert at one small aspect of a larger thing which is not necessarily youth-oriented (so your interest in it can continue). Whether you watch all the movies of Stanley Kubrick or analyze the horror movies of the 1950s, learn the Latin names of and how to recognize plants and trees in your region, recognize the paintings of the masters on sight and learn their names, or can compare and contrast the literary themes in all the Dickens novels or Shakespeare plays, become knowledgeable about something few people are, so long as you start out with some basic interest in it. (One of our kids spun this into a college application essay which seems to have worked.)
Take up a craft. A small monetary expenditure, some online tutorials, and the time to experiment and risk failure can move you far past amateur status in whatever you're interested in. You can sew or knit—not just for the ladies, you know—bake bread or make soup, cultivate roses or grow your own tomatoes, etch glass, work leather or metal, carve wood, refinish curb-find furniture, make toys, teach yourself computer programming, dip candles, make candy or paper, weave or dye cloth, learn beadwork, build radios or wooden boxes, stencil borders on walls, “paint” with moss, or any of a bazillion other crafts. (Visit Etsy and Pinterest for inspiration, and Intructibles for how-to.)
Become an artist. You can draw or paint, write poetry or novels or music, master playing a new instrument, make complex quilts or torn-paper art, get into photography or printmaking, work with clay or video, etc.