Well Resistance for one if they like FPS games and online multi player, better be quick though with all the trouble its getting at the moment. The game starts off a bit slow at first but it gets a lot better, one of my Fav FPS games on any format. The multiplayer is great 40 player online death match is great fun if a bit too fast paced sometimes.
Motor storm is a good game if a bit limited in the options department. Online quite fun and there is some updates coming out soon to improve the online game.
F1 Championship Edition is great, but i would say its for F1 fans mostly as it is quite in depth but even so if youre not its still a great game to play not really tried in online much though.
If they haven't played Oblivion and like RPG type games then this is a must so much to do and is a good way to waste most of your life away!
Virtual Fighter 5 is a very good beat em up which will take an age to learn completely, but is still good fun, no online though which for some is a problem. Tekken 5 DR on the PS network is also very good and worth the £7 it costs now even though its a very basic option wise beat em up.
Towards the end of the month Ninja Gaiden Sigma is out if they havent played NG Black on the Xbox. It is a remake with extra levels weapons and playable characters though, is a bit hard though!
If i was to pick one it would be Oblivion just for the shear size and what you can do in the game, if its not there thing cant go far wrong with Resistance Fall of Man.