If your dad enjoys and little mystery or a treasure hunt, try putting it somewhere then write up instructions or a map to locate it. Put this in a box, maybe inside another box, that is inside another box. Or one clue to another clue to another. Be creative.
I did something similar a few years ago for my mom.
She was recovering from shoulder surgery, she was a little down because she could not get around much. Needed a little pick-me-up.
She's in Green Bay, I'm in Florida. She's always enjoyed poetry, puzzles and mysteries. And her favorite snack in Pecan Sandies. (bear with me)
So I sent her a locked box with no note or anything. Inside was a box of Pecan Sandies.
A few days later I mailed her a mystery poem: An intricate affair involving family members, dates, vacation memories, and some historical references. All very personal stuff that only she would understand. Once solved, it would tell her what was in the box, but not where the key was. She had to further analyze the entire mystery to decipher where the key was.
She had a blast, I think she said she spent like 40-50 hours trying to solve it, over a two week period. Well it sure took her mind off her recovery. That was five or six years a go and she still talks about it.