Good suggestions with a wide range of genres, pr2thej. Although for me I'd go with the new PoP over Assassin's Creed. If you like adult language, lol...Fallout 3 may be just the thing for you.
Titan Quest was a good single-player timesink, and the Gold version of it with the expansion pack is only $20 - heck of a good value. Hellgate: London is decent, too, considering you can pick up a used copy from Amazon for like $3 + $4 for shipping.
Mass Effect may also go in that list of my favorite games of all time...it has shooting elements like Fallout 3, but really can't be classified as a "shooter," so it shouldn't make you nauseous.