My last campaign featured the PC's (near the end of the campaign) trying to protect a valley from an imminent attack by combined forces of Duergar, a clan of Half Orcs, and some re-skinned Yaun-Ti.
Well, one day we had some players suddenly (the day before) cancel so I only had 3 players. So what I did was made up 3 new characters... a Duergar, and two Half Orc brothers.
They were to be sent on a mission into the valley to ransack a tower that "was of some value" regarding the defense of the valley - they didn't know what was in, but only that they needed to find out what was there, and destroy it if it could be destroyed. They were to go in just the 3 of them so as to not draw attention to themselves - it would be a stealth-ish operation.
This was a very fun game - they moved through the tower from the top down (the tower stood in a deep river, the top barely exposed). At the bottom, they found an exit into the river (magic held back the water) which allowed these people access to a colossal construct underwater that was being re-assembled/powered.
They had 3 encounters on the way in (not counting a few traps) and one final encounter in a big chamber (with the opening to the water). Here they fought humans wearing mech-type armor (that they used when out underwater). It was a great fight, and they were able to rig the room to explode, getting out of the top and away from the tower in time to 'feel' the explosion underwater and watch as it ripped its way out through the top.
It was a lot of fun and it gave them some insight into the other side... a view of what their enemies could do.
So you might consider something like that since you have been running a campaign for them and this is a diversion. It makes it a lot of fun (one-shots) when you can show them something of their world from another angle - really makes it feel more alive - like there are things going on even if they can't see them
