Build this step stool with a single 8 foot long 2×4, without using screws or nails. See how at the link.
This squirrel feeder projects is easy to build and only required a six foot fence board, a handful of screws and a 1-gallon glass jar.
Scrap lumber of nearly any kind can be used to build a squirrel den box. Exterior-grade plywood may be used, but squirrels may destroy the box by gnawing the plywood.