Here is my two cents:
As for whether to buy tickets on StubHub or wait to buy them on game day from a scalper, I would go with the former. While tickets for the Miami (OH) game are likely to be easy to find on game day, this sounds like one of those bucket list trips for your pops and you probably don't want to mess around with looking for tickets on game day so that you have time to take in all the other experiences. Take in the sights and sounds and don't worry about having to find tickets. While it will probably cost you a little more, some day when you are looking back on the weekend you wont' regret having spent a few extra bucks to share a day with your father that you and he will never forget.
I would strongly suggest getting to South Bend on Thursday so that you have all day Friday to look around the campus at the various places your father might want to see: the Golden Dome (admin building); the Basilica of the Sacred Heart; and the Grotto. There are likely many others. If you arrive on Friday you will probably feel rushed trying to take it all in. The football game is almost certainly going to be an afternoon game, so there will be limited time available on Saturday for touring campus. Getting there on Thursday also gives you time to make the Bookstore visit on Friday. (I assume your pops may want to pick up a souvenir or two and maybe some ND swag.) If you wait until Saturday to do the Bookstore trip the place will be crowded and the lines long.
The university always puts out a schedule of activities for the weekend. Check the university website (
https://fightingirish.com/fbgameday-traditionsandevents/) a few weeks before the game. The drummer's circle is always popular, as is Trumpets Under the Dome. You may want to roam the parking lots on Saturday for a bit to check out the tailgating. That is a fun part of football weekends. You will likely find a USMC flag flying somewhere, and your pops can make connections with a fellow jarhead. Once a jarhead, always a jarhead. The brotherhood runs deep.
You will have a great time. Good luck.