What can I make out of scrap lumber?

For the 2X stuff I can't add much to what has already been suggested. For the plywood, make birdhouses. You'll want to nail and waterproof glue the connections. Sand well, and THEN fill gaps with waterproof caulking. Then paint with opaque deck stain. This will hide the fact that you're not using better wood.

How about little stools, side tables, small trash cans, small flower boxes, you can do lots of things with little pieces of wood. Make a shoe shelf.

doghouse

BONFIRE!

Storage bins....you cannot have too many of those.

I would try to create bird houses from the various pieces.

you would be suprised theres a guy on the golf channel called Dave Pelz and he made a putting robot out of scrap its a pretty pointless thing but then again so is Dave Pelz

Well, at 24" lengths you're pretty limited as to what you could create. One thing that comes to mind is an end table or coffee table, sort of on the rustic side. I have made plant stands with similar material that have turned out sturdy and efficient for larger plants, add casters at the legs and you save your back when moving them. I made the plant stands to look like mini picnic tables and painted them. On an end table you could use the ply wood for the top, sand it real smooth, add trim along the edge and it'll look like a solid finished piece. Good luck

a campfire? Just Kidding . . . How about a butcher block, would be great practice in joinery . . .