Working out at home is convenient, and the price is right. But if you’re lucky enough to live in one of those $200,000-a-month Manhattan studios—or if your kids and their junk have oozed into every square foot of your once-spacious home—you may not have room for all the ski-fitness equipment you desire. Fear not. Here are four items that are easy to stash and serve up everything you need to stay in prime ski shape. The first three are also ideal for travel.