I modded my A1UP pacman, and added 1 button. I moved the joystick to the left and added a button where the joystick hole is. I have a couple game suggestions:
If using 2 buttons, one of my favorites is Astro Blaster
If you can deal with a horizontal screen: Tapper and ZooKepper are both a lot of fun
If you can deal with single player only and a horizontal screen: Joust and Mario Bros
Some other decent 4 way games: Eyes, Ladybug, Crush Roller, Bomb Jack, Qix and Mr. Do (Castle is good, but I like the original a bit more)