I'll second Neverwinter Nights. It'd be really fun if you guys got good at building your own levels, too. Then you could challenge each other or something.
When Diablo 2 was just about to come out, Darkstone hit the shelves. In my humble opinion, it's better than D2, but admittedly, I just couldn't get into D2. Darkstone was my baby. You play the game co-op, but occasionally, you can turn off the friendly fire and duke it out. You can play a lyncanthrope witch, a rogue who can get so good at stealing that you'll pull eggs out of chicken's bums, or a healer or warrior. Each class has a male and female type, but the only differences are in the wizard (one gets lycanthrope and the other gets ?detect magic? -- I actually preferred the male more, but the werewolf is a good time, too).
If she will do a military sim, I've been having a lot of fun with ARMA II. But that's definitely not everyone's cup of tea.
Diablo 3 could be a good one to get into. The new loot patch has made the game a ton more fun (I liked it before, but it was getting old after 50 hours). You'll get loot that will boost one of your skills by a fair amount, which will more or less force you to change your build and your playstyle. It keeps you on your toes and gets you to experiment a lot. It's really fun.
IL-2 is awesome co-op (or vs). As is Rainbow 6.
BattleRealms is a unique RTS worth a go if you like that sort. And Empire Earth can waste a whole evening in no time.
Magicka Titan Quest is a good action RPGs -- the former very funny and the latter very serious. And Heroes of Might and Magic is a fun turn based strategy that you can play vs or co-op. And I'll second Sacred 2, also. So far, I'm enjoying it (though Thief has led me away for a bit :D).
Other games that still get lots of LAN attention for me (and my brother) are:
- Star Wars: Empire at War
- Civilization IV
- Need for Speed Hot Pursuit
- Splinter Cell: Blacklist
- Rainbow Six: Vegas 1 and 2
- IL2 (as mentioned above)
- HAWX
For my wife and I, we play very different games. We might do Peggle, Wheel of Fortune, Jeopardy!, or play over-the-shoulder Sherlock or Drawn games.
Have fun!