A few years back, I built a twin version of Ana's Farmhouse Storage Bed for my step-daughter. After she moved out, I converted it to a huge toychest for my three-year-old.
I would have left it as a bed, but with a quality mattress it was so high I was afraid he could get hurt if he fell out.
The conversion is simple. Remove 24 screws, reconfigure, add back the screws in different places. The support blocks become feet for the chest, and the slats become spacers under the top part.
He loves being able to get his toys out himself, and I love that he can put them away by himself. A big upgrade over plastic storage boxes on high clost shelves!
Once he's a bit older, I'll probably turn it into a bad again.