Beautiful Katamari's dual player is terrible. Don't...
One of the best casual games in my opinion would be the Lego Series. Lego Starwars and Lego Indiana Jones are fun and easy to play. They are co-op same screen and no-one ever truly "dies".
As mentioned above, Earth Defense Force. It's a bit cheesy but it is really fun playing split screen co-op and on Easy, it's a walk in the park.
The last thing you *might* consider is to watch for a 20 gig hard drive on Ebay or Craigslist and see if you can find one for 20-30 bucks. You can then download demos from XBLA for every game on there and find some that will be great in co-op.
Halo 3 is local co-op and on "Easy" is also a walk in the park and a lot of fun to play.
Rainbow 6 Vegas is a good co-op.
My favorite "Pick and Play" shooter is actually on XBLA and is called "Assault Heroes". There is a II now as well. It's an overhead, modern version of Ikari warriors and is completely co-op. For an XBLA game, the graphics are awesome.
Is there a particular Genre you are looking for?
I hate trolls.
Systems I currently own: 360, PS3, Wii, DS Lite (2)
Systems I've owned: PS2, PS1, Dreamcast, Saturn, 3DO, Genesis, Gamecube, N64, SNES, NES, GBA, GB, C64, Amiga, Atari 2600 and 5200, Sega Game Gear, Vectrex, Intellivision, Pong. Yes, Pong.
