Of course, they needed to actually make something, so he prepared the parts they needed to make a wooden puzzle - The Tower of Hanoi.
Step 1: What you need.
The boys used 15mm-thick MDF for the base, 5mm thick MDF for the puzzle-pieces and 5mm dowel for the posts.
The tools they used included coping saws, tenon saws, scroll-saws, files, glass-paper, a pedestal drill, power-sander, band-saw, circular saw and a mallet.
Oh, and goggles.
Step 2: The base.
The holes need to be around 50mm apart to allow the puzzle-pieces to sit side-by-side.
Using the pedestal drill and a 5mm bit, drill the holes about three-quarters of the way through the base. If you are able to, set the stop on the drill to prevent it going all the way through.
After drilling each hole, knock the dust off the base, don't blow it off.
You will, of course, need to wear goggles.
Step 3: Shaping the base
They drew outlines on the bases, leaving enough wood around the holes for strength.
They practised using a coping saw and a tenon saw on scrap wood, and decided they would rather use a power tool to cut out their proper bases.
So, roll out the scroll saw. Goggles on again, make sure the guard is placed to allow the base through without finger, and let Grandad hold the base firmly while you guide it through.
Step 4: Filing!
Time to break out the files.
Files were popular - as noisy as a power-tool, without having to worry about where the emergency stop button was.
Step 5: The posts.
To help them fit in the drilled holes, and to allow air to escape*, he added a slight chamfer with the disc-sander.
The boys then smacked them into place with wooden mallets, and friction held them firmly in place.
* I didn't realise, but apparently square-ended posts trap air in the hole, which is compressed as the post is pushed in. This pressurised air can split the base. You can get away without doing it, but not if you are using glue in the hole as well.
Step 6: The puzzle pieces
The boys had to find the centres, drill holes fractionally larger than the diameter of the posts, and then smooth the edges and corners, and remove their pencil marks, with glass-paper.
Since the pieces were too small to clamp, they used a small hand-vice to hold the MDF during drilling.
Step 7: The finished product
Both boys had, in about ninety minutes, turned scraps of wood and MDF into proper, usable puzzles.
They had experienced tools and techniques that most children don't get to use until they are years older (#2 son is seven - most UK schools restrict use of scroll-saws to age 13), and learned how to use them safely (they also had a go with a plane, and watched Grandad use a band-saw and a circular-saw).
Most importantly, they had a great day with their Grandad, and took another step towards getting their own accounts on Instructables (surely the pinnacle of any Maker's career?).
Some background to the puzzle.
If you don't want to build an actual tower, you can attempt the puzzle here.
I didn't do an awful lot except take photos, and help out with some of the more boring sanding (who can sand things when Grandad is using power tools taller than you are?)