SC: Conviction is a fantastic game. I know you said that you're not fussed about the MP but that game's brilliance is extended through the MP. It must be available split screen though (correct me if I'm wrong) so you could probably still avoid XBL and get the most out of it.
I think FORZA 3 is worth checking out too. I preferred that over GT5. If you don't have Mass Effect 2 on the PS3 or tried 1 then that too is really worth picking up but be warned, ME1 can be a bit rubbish at the start until the story kicks in.
I haven't played Alan Wake myself but a friend has recommended it and said it is worth playing. I'm a fan of all Fable games but I know that series can be an acquired taste.
Not sure if you are in the UK or not but I was in GAME on Saturday and it's always worth having a look through the pre-owned shelves, I know you can get Crackdown 1 and 2 pretty cheap as well as a few others. Obviously that will be based on a store to store basis.