Children go through different stages of discovery. Our son started throwing toys and pushing toys under doors and such then peeking under to see where they were and why they didn't come back. It's a spacial relationship and develoment milestone.
They are discovering noises, cause and effect and that objects exist even if they don't see them. (object permanence).
Our son started learning this a few months ago when he threw food over the side of the high chair but realized it was still there when he looked over the edge. Or saw his Hotwheels still existed even though he pushed them under the closet or bathroom door. It's how they learn about objects, sounds and the world around them.
You may limit the noise or damage to your home buy buying soft balls he can throw down the stairs, but these may not be as fun for him. I agree with other moms on the fact he isn't too young to pick the toys up himself or at least start by having him help you pick them all up.
When our son started to enjoy throwing toys we got him two sets of balls from Target. One set was colorful raquetballs, I think there were 20 in the set for $5, also a great tool to teach his colors. And another was a small rubber set containing a soccer ball, basketball and football, also $5.
Just the right size for his small hands and feet but not made of metal or plastic that would hurt furniture, the walls or the baby.