Thanks guys. I appreciate the responses.

I think I'm leaning towards NSMBW after your opinions.

NSMBWii is also great because you can play it with him whenever you're over. I can't really recommend any game more highly than I do that one.

If he is still quick on his feet and active, Punchout is cool.

Don't  forget Virtual Console too. Super C

You can't go wrong with New Super Mario Bros., but I'd say the same thing about Tiger Woods.  Basically, if your dad loves playing golf but can't make it to the range as often as he'd like, Tiger Woods is the better choice.  Otherwise, go with Mario.

Endless Ocean 2

If he likes puzzle games, Tetris Party Deluxe is a great option.

plus 1 recommendation for Tiger Woods

As for NSMB Wii I would consider that more hardcore...sure it can be played casually with multiple people but may he would find it too daunting as a single player game....just another opinion.

A way out there suggestion would be HotD: Overkill. It's really fantastic and really simple to play so it meets the casual requirement. The only thing that might turn you off this idea is if your father is a bit conservative or prudish he might find it offensive but otherwise I think this would be highly recommended. It also would work well if your dad likes shooting.