Two bits of paracord, a lump of technoscrap, and some Scouting know-how, and you have a gift fit for any cool dad.
Step 1: What you need.
I used a ceramic thing that came out of a computer, but you could use anything that's the right size and has a hole in it. Maybe cut a piece of wood and drill it.
Step 2: How to make it (short version, for if you know your knots).
Fold them in half, and tie them to the thing you're using with a lark's head hitch.
Fasten the double ends together with a Fisherman's knot.
Step 3: Lark's Head Hitch
Put the looped end of the cord through the hole in the whatever-you're-using.
Put the loose ends through the loop, and pull it tight.
Step 4: Fisherman's Knot
Tie the first cord's end (except, for the bracelet, it's two ends together) into a simple over-hand knot around the second cord.
Tie the second cord's end in an over-hand knot around the first cord.
Put the bracelet around your dad's wrist, and slide the two over-hand knots apart to make the bracelet tight enough to stay on his wrist.
If you don't like the simple version, you could try the Double Fisherman's Knot, but it might be fiddly tying doubled pieces of cord together.