I haven’t played Children of Mana co-op, but it is a fantastic game. Really, really fun. If you’ve played whatever Mana game was on SNES, then I’m sure I’ll get a reply about how this isn’t a real Mana game and whoring and driving a license into the ground and blah blah blah. Otherwise it seems like it would be a very fun and long cooperative 2-player game.
Animal Crossing Wild World. Its a huge time suck, although it doesn’t exactly have co-op play. Come to think of it you can have fishing contests (very slightly adversarial) and you can visit each other’s towns and swap fruit, help each other out with furniture etc. That’s pretty co-operative.
Animal Crossing is a great game, but the actual multiplayer aspect is pretty minimal. Most of the time you’ll just be playing the same game separately. But if you’re looking for a time suck, there are few better.