For the super-hero-loving toddlers, this kit comes with Force-Field Fort Sheets (including a "superhero secret entrance" printed pillowcase), a trusty Rescue Rope, Emergency Flares (glow sticks!), a Beckoning Signaler (flashlight!), and a Secret Lair Flag.
If you'd rather go the store-bought route, Crazy Forts is a collection of sticks and balls, which can create virtually any type of fort. My son has this set, actually, and it's always a fun rainy-day activity.