Set of Dumbbells
“Dumbbells give your workouts variety,” says Rachel Cosgrove, author of The Female Body Breakthrough ($22, amazon.com). And they can also challenge your muscles more than gym machines do because you have to stabilize yourself to lift them. Start with a basic range—say, a set of 5-, 8-, 10-, and 15-pound dumbbells (sets cost about $5 per pound). Increase the weight as you get stronger.