Tweens are notorious for being a fan of one thing one week to totally flipping and loving something else the next. Shelving and under-bed storage leave room for them to store things until they're "cool" again.
If you don't have much space to work with, concentrate on building up instead of building out. These floating shelves, for instance, are stylish and practical. What makes them even better is their budget-friendly price tag—each one came out to just $40 in wood.