If you want to give your credit card some exercise, head out to Santafé Mall in El Poblado. It's also full of great shops and restaurants, so you can spend the afternoon having a wander. Splash some more cash at Oviedo Shopping Center, Golden Mile, and La Strada Mall.
You can spend an afternoon browsing the exhibits at Medellin's Museum of Modern Art in MedellĂn. Attend a sporting event or simply enjoy the lively bars while you're in the area.