You're right that Sanford is too far for us but I think it will be easy for you to get you Locker back to us. When you are returning from Anna Maria, which is on the west coast of Florida, you'll be passing right by WDW on your way to the airport and you can drop your locker off at any resort (you don't have to be staying there). Pop Century is probably the closest resort to the I-4.
You will absolutely love Anna Maria. Spending time on the beach is my favorite way to recharge my batteries, and Anna maria and Longboat Key are my haunts. If you don't mind eating at dives, check out Skinny's for the best burger on the planet...and at the complete opposite end of the spectrum is Beach Bistro (bring lots of $)!