(When you say no online, do you mean no Gold membership or no online access period? My post is going to assume no Gold membership. If you mean no online period, I highly recommend finding a way to at least temporarily access the online service for demos and XBLA games.)

Mass Effect should be your first priority. Get it now.

Even though you don't like first-person shooters, download the BioShock demo. The demo is very well-done and should tell you if the game is for you; it's much more than just an FPS.

For action-adventure, Assassin's Creed and Prince of Persia are both love-it-or-hate-it games. Look into them and see whether either one might be for you. Personally, I greatly enjoyed both of them.

I see you already have Gears of War. If you have any friends who enjoy shooters, I highly recommend playing the split-screen co-op. It really takes the game to a whole new level. You also have Fable II; my only recommendation there is for your wife to try it out. I know several women who aren't into RPGs, but absolutely love Fable II.

As for XBLA, I recommend checking out the trial versions of Braid, Geometry Wars: Retro Evolved², Pac-Man Championship Edition, and Prince of Persia Classic. It also looks like you don't have Puzzle Quest on one of your other gaming systems. If you haven't tried it out, do so; both you and your wife might enjoy it.

lost odyssey
blue dragoon
tales of vesperia
last remnant
star ocean 4: the last hope
elder scrolls 4:oblivion
sacred 2:fallen angel
gears of war 1 and 2
viva pinata
kameo
banjo kazooie nuts and bolts
fable 2
resident evil 5
dead space
mass effect
tomb raider underworld
virtua fighter 5 online
dead or alive 4
soulcalibur 4
forza 2
burnout paradise
orange box
condemned criminal origins
riddick
enchanted arms
culdcept saga
splinter cell :double agent
ghostbusters
ninja blade
halo wars
marvel ultimate alliance
star wars force unleased

For tps i recommend you Red Faction Guerrilla!Try the demo, it's very nice.

Now you really R2 hardcore!

ha ha, ha ha ha.

The Wii was the path to the dark side for my wife and son. Now they are never coming back!

Mass Effect > best sci fi RPG of all time

You should get this driving wheel with force feedback, it's pretty awesome.

http://www.amazon.com/Xbox-360-Wireless-Racing-Wheel/dp/B000VBIGK4/ref=sr_1_1?ie=UTF8&s=videogames&qid=1245639548&sr=8-1

Ok, then i can recommend:

TPS: Resident Evil 5 (it's more a TPS than a survival-horror action game, but you like both so you're covered either way)

FPS:

-Call of Duty 4: Modern Warfare (seriously, it's that good, Activision had to separate this title from the CoD Franchise and made it a completely separated franchise, Modern Warfare 2 comes out this fall)
-The Orange Box, its a pack with Half-Life 2 original + Episode 1 and 2, Team Fortress 2 and Portal (I got it thinking in Half-Life Episode 2 because i played the original and Ep 1 already, but the game that got to me was Portal, its an amazing first-person action/puzzle, look at the previews on Youtube, it's really good)
-Bioshock: Really, the game has a cool storyline, a great experience and if you're into graphics look at the water in this game...

RPGs:

-The Elder Scrolls IV: Oblivion (Bethesda Softworks is a long time RPG developer, their games started on PC, and they added consoles when they became powerful and profitable enough, and this is a great example of their amazing work)
-Fallout 3 (The previous main titles you're going to find them only on PC, but this title is completely different from the previous ones, because Bethesda bought the Fallout IP once Black Isle was shut down when Interplay filed bankrupcy, and they made the whole game from scratch)...
-I forgot about Fable II, this is a game that you and your wife can enjoy together if she likes it (a lot of women like this title)...

Racing:

-Midnight Club: Los Angeles.
-Burnout Paradise, good combination of both racing and sandbox genre...
-Need for Speed Undercover (the game has enough racing action to get it, just don't pay too much attention to the cutscenes)

Welcome aboard the Xbox boat of love!

Enjoy your stay.

Thank you for your time

Just a question... Which one is better?

 CoD4 MW?

The Orange Box?

Bioshock?