As an avid electronics hobbyist, my kids grew up with a different workshop than I did - one full of circuit boards, cables and computer components. So before they were old enough to make "real" electronic projects with me, we started by making our own version of the venerable whatsit board. Many of them started out with the same trusty substrate - a hunk of wood - but a few have been fashioned out of old project enclosures. For the most part they weren't functional - the kids would happily use their imaginations to talk with each other using their homemade "space communicators". Along the way we made a few with real working parts - a few LEDs connected to some batteries, or an old CD player with a tiny speaker wired into the headphone jack. My younger kids still regularly stop into the workshop to help take stuff apart and make new toys together.
The Whatsit Board doesn't really warrant a step-by-step Instructable - all you need is some old electronic junk, a scrap piece of two-by-four, a few hand tools to take the old stuff apart, and a LOT of hot glue. NOTE: I recommend using old "low voltage" items that don't plug into an AC outlet - if you're not experienced with electronics, an internal AC power supply could discharge current while being disassembled, and that might not be fun! Most small electronic items that run on a "wall wart" power adapter or batteries will be a lot safer. Once you have some old circuit boards, knobs and displays worked free, just let the kids arrange them on the board, and then you can hot glue them into place. On occasion we've used a few wood screws when the old parts had mounting holes that could be reused, but it's not really necessary. As a final touch, take a few pieces of wire or ribbon cable left over from taking stuff apart, and "interconnect" some of the parts to make them look like they really do something.
Hope you find this fun to do - we've always found it to be a great way to recycle old items while also encouraging some creativity!