For my wife, I pick fun or cute things like LittleBigPlanet or go with older characters you recognizes like Mario. She also likes bright and pretty games with pleasant soundtracks, so she's a big fan of Final Fantasy.
Things that are out:
Horror games
FPS titles
Things with too many explosions or annoying noise (few exceptions)
Things with frequent non-English voice acting (Zelda, Japanese voice acted games, etc)
It'll be different with every person though.