There are now seven of us players in the game, although the two new players haven't actually had a session with the rest of us. The DM's whole goal adding players was to avoid having sessions like this :P Well, at least by playing on Roll20, no one had to actually drive to someone's house to find folks missing. (And, in the defense of two players, they gave definite no's ahead of time)
So we ended up having a side quest in the middle of our fetch quest. Although this side quest might end up having repercussions that last far longer than our fetch quest.
Short version: the three of us stumbled upon the tomb of a elven king. It helped that the entrance had been cleared by some now dead tomb raiders. We weathered some encounters with a lion, skeletons and grey oozes without too many serious problems.
Then we opened up the tomb of the king and unleashed the mummy that he had become. Three second level characters against a CR 3 monster isn't an instant kill but we were clearly outmatched and our healing couldn't keep up with his damage. Fortunately, the dwarf cleric had his daughter's ring (we found it in a hidden fountain) which make the mummy think the dearf was her. He convinced the mummy to leave us alone and leave. We were hoping the lion we had left alive would do enough damage to the mummy that we could take him in a second bout but the lion ran away.
So, we ended with a royal tomb's worth of treasure but we had unleashed a relatively powerful undead onto the countryside.
Mechanically, this session let us explore ideals, bonds and flaws. Basically, we knew opening the king's tomb was clearly a bad idea but of course we wanted to. Looking at our character aspects, which reflected the pseudo Viking culture we were a part of, glory and treasure were too big a driving force.
Storywise, it's fun to push a character's flaws. Yeah, we were a bunch of greedy idiots but that made things a lot more interesting. And this side quest which we played because we were missing too many players may actually create an ongoing villain who could be a part of the game for a while.