Our group usually puts together a patrol scenario from Star Fleet Battles. I can occasionally get them to do Battletech. Car Wars comes in to play as well once in a while.
However, I am the DM as well. Usually I have to find something for the short group to do as opposed to them finding something to do if I cant make it. As a matter of fact, I host the games at my place, so if Im not there, no one is.
This time around I have Iron Heroes...I think Ill draft a few simple solo missions. We have an executioner who plays the assassin that just begs for mission of its own. Course I have to do something for the rest as well to make sure they dont fall too far behind the curve if he's out taking missions.