There's a lower-level quest you can do called the chain gang. It's on the wiki for quests in NCR. *note: you can do this solo at higher levels or with good equipment, so it's best if you and some friends do it at the minimum level for a challenge.
I think I read somewhere there are some new co-op quests planned for down the road. If anyone has a link to some info on that we'd all appreciate it.