Well Tetris DS would be a good idea as it's a good thinking game. IMO it uses the same sorta stuff as an RTS (lol I'm kinda serious). There is a good lot of thinking involved, plus you need the speed if you wanna be good. There's lots of modes, like a touch mode and a puzzle mode where you work stuff out, that your dad might like.
Or a real cool strategy game is Advance Wars: Dual Strike (although AWDS2, will be coming out some time soon, so you might wanna hold off). It's a turn based strategy game where you build units and move them around and attack and stuff.
Those are the two best games I can think of that are for thinking, other than the brain training games of course. Good luck with picking.