If your husband would rather have the guitar that comes out in January then I suggest you wait for that.
The Orange Box is awesome and will keep you entertained for a long while. Also Bioshock looked awesome, I only played the demo and haven't bought the full game yet but it looked brilliant, and most people I've spoken to about it loved it. (Tis scary though >.<)
COD4 is a really good game, you might not like that type but you seem pretty open minded to trying out new ones so give it a go really, it's amazing. The most obvious game to suggest also is Halo 3, it is short but I really enjoyed it, was a lot of fun.
Can't really comment on much else, Command and Conquer 3 is a really enjoyable game if your into that type of games. I picked up Tomb Raider and Crackdown yesterday but haven't even opened the cases yet, (been moving house AGAIN raghr kicked out) but I'll be able to give a decent opinion on those by the end of the week probabaly, or I'm sure someone else on here could.
There's loads of other people on here who can tell you much more than I can lol, I've only had my 360 for about 2-3 weeks, but I have played the games I've mentioned so I'm not just making this stuff up lol. Good luck ^_^