I admittedly got this idea from one of the 2 year old classrooms at my school. The kids were studying all different ways to use boxes. The teachers taped some big boxes together to make blocks for stacking and building. I thought the idea was great and since O loves to stack (or rather he loves to stack with the intent to knock over), I knew he'd be into these big blocks.
I collected a number of boxes we still had from deliveries from Christmas gifts.
I taped them up early one morning and piled them high for O to discover when he woke up.
He quickly got to work disassembling my tower.
And then building his own.
And of course knocking it down!
These box blocks are great because they are big enough to encourage some gross motor development (wide arm movements, lifting over head, kneeling to standing position while carrying) but not too heavy for him to move around. O stacks them and knocks them down constantly, tries to balance the bigger ones on top of the smallest, uses them to stand on, and even as a small table for puzzle work.
For infants and young toddlers it would be fun to use really small boxes for them to stack or build with, offering them bigger and bigger boxes as they become more mobile, and learn to stand and walk.