X-Men Legends would be a good choice, but it IS an RPG, so there may be the need to do some grinding.
Shaolin Monks is also a good possibility, but I hear the end is a bitch. You could go with the second or third Lord of the Rings games too. They are a lot of fun with co-op. Or you can just go with a completely mindless game like Dynasty Warriors or Guantlet.
It's not an FPS, but it's a shooter. The Star Wars Battlefront games are great fun, and not too difficult. The first one makes you feel like the ultimate badass with how many people you can mow down, while the second one is a little tougher. But playing with two people is great fun, aside from the whole splitscreen thing.
Check these out too;
Check these out;
http://xbox.ign.com/articles/535/535509p1.html
http://xbox.ign.com/articles/538/538745p1.html