Another vote for Rayman but consider making future plans to purchase a Wii U :p

Resistance 1 and 3 specifically 3 is good for all the great weapons. Obviously the Borderlands games. Playstation allstars battle royale when you do 2 vs 2 it can be a blast. Tetris on ps3 has a co-op mode where you have to clear the same line together but are limited on what area you can drop blocks. It can be really fun you can trade pieces with each other say when one of you needs a straight piece, etc.

Yup! That's a huge system-seller right there! No, I'm NOT being sarcastic!

You people saying spin the bottle strip HA! you dont think shell be topless anyways?

I think the best one so far is Portal 2 didnt even cross my mind, also i didnt even know rayman had multiplayer :P

On the flip side is Modulation Racers any good, and how does SSB go against Playstation all stars?

Both Rayman Origins and Rayman Legends have co-op! I recoomend Legends because it's better in pretty much eveyr way possible!

Get a Wii or Wii U, then pretty much every game is multiplayer.

But as for PS3 games? Um.... all I can think of is;
Minecraft
LBP
Resident Evil (but may be too hard)
Borderlands 1 + 2
Some other shooters are Co-op but depends if you both like that genre

Um... that really is all I can think of right now.

OH my favourite co-op PS3 games are Warhawk and Pixel Junk Monsters, the 2 best multiplayer PS3 games ive ever played.

Metal Slug
Shock Troopers
Moon Divers ( 4 players)

Some great suggestions have already been given, and I will back some of them up.
Portal 2
Borderlands 2
Playstation All Stars
Rayman Legends/Origins
Lara Croft + Guardian of Light
Ratchet and Clank: All 4 One is pretty fun

Singstar or Sports Champions if you're willing to dish out that extra cash.

I know quite a few girls that like RPGs so if you can find a co-op one then that would probably work rather well.