A good project to start with is a sturdy workbench. Build two large rectangular frames, 24 by 48 inches each, one for the top and one for a shelf. Cut legs 36 inches high. Cover the two frames with 3/4-inch plywood cut to fit. Notch the corners of the shelf plywood before attaching it to the frame and slip your legs through the notches. Attach the shelf to your legs about a foot off the floor with long drywall screws. Fit the top onto the legs and screw it in place to create a perfect work space for your other projects.
Create extra storage by building a temporary wall to divide a room with 2-by-4 framing. Space your studs 16 inches apart and attach them to a horizontal 2-by-4 at top and bottom. Use doubled 2-by-4s on each side of your door, and use them doubled on edge above the door for a header. Make your frame a little shorter than the ceiling. Attach the wall to the wall studs at both ends and cover it in plywood or pegboard. To remove the wall, if needed, just detach it from the studs and patch the drywall.
Rip 2-by-4s to 3 inches wide on a table saw and cut squares, rectangles and triangles from 2-by-4s in multiples of 1 1/2 inches for a set of geometric blocks. Make plenty of each shape to build a fantasy castle. Use a miter saw to cut the pieces to size. Add arches to a few with a band saw. Sand the edges extra smooth and use only nontoxic finishes, since small children will shove almost anything into their mouths.
In the yard, 2-by-4s have a multitude of uses. You can build trellis frames and attach lattice to them for growing roses, climbing vines or beans. Laid on edge, 2-by-4s make great borders for small flower beds, or to contain bricks and thick pavers for custom walkways. Use treated 2-by-4s for projects that will be exposed to the weather, especially if they will be in contact with the ground.