Here is a master list of my recommendations for your dad in no particular order:
-New Super Mario Bros. (similar to the original, classic side-scrolling platformer)
-MarioKart DS (a fun "racing" game, good for all ages)
-CrossworDS (crossword puzzles with excellent touch controlls)
-The New York Times Crosswords (more challenging than CrossworDS)
-Peggle (a pachinko-like puzzle game)
-Bookworm (a boggle-like word puzzle game)
-Professor Layton & The Curious Village (adventure game with puzzles)
-Professor Layton & The Diabolical Box (sequel to The Curious Village)
-Hotel Dusk: Room 215 (point-and-click, novel-like mystery)
-Tetris DS (if you can find it...beware of bootleg ebay copies)
-ClubHouse Games (card and parlor games, darts, ect.)
-Mystery Case Files: MillionHeir
Now obviously there are many more games I could recommend but here is a nice list that depending on your father's gaming habits, should keep him busy for a while. Most of these games should be easy to find, except maybe Tetris DS. Hope this helps!