Recently I added a simple stop watch to my sunday Labyrinth Lord gaming sessions. I set the timer for 45 minutes and start it going at some point during the game. Whenever the alarm goes off - no matter where the heroes are or what they are doing, I roll for wandering monsters.
This has led to some interesting encounters as you can probably imagine. One time the heroes were battling carrion crawlers in an abandoned temple and suddenly a bunch of skeletons and ghouls joined the fight.
I like the unpredictability of approaches like this. Imagine if the heroes were in a tavern in town and the alarm goes off. I have to work that into the story some how. Did the skeletons suddenly break through from the cellar, or is the town itself under attack? It is good to stretch yourself as a DM a little with things like this I find.
I've been thinking about expanding this idea from just wandering monsters. Sometimes in game sessions there is a lot of fighting already. On the previous game session, for instance, we had a pitched battle against fifteen bugbears because the heroes did not want to pay an extortionate amount of gold to cross a bridge and the following combat was against four efreets. In that session, I did not want to lengthen the combat further with more monsters arriving. So sometimes there is a need to do something else with the wandering monster results.
I have a trap dice and I think I will start including that in my wandering monster rolls, essentially, having the party run into a trap. Traps are encounters too, especially if they may be detectable before hand from either a thief or a clue in a description and can be fun to disarm or work around... providing they are of course not deadly.
This got me thinking further. As one definition for the word encounter is a problem, I wonder what other types of problems could be thrown the heroes way on a wandering monster roll. Social problems, the ones that you must talk through, spring to mind, and would require considerable thought but could be fun. I wonder what other classes of problem would be fun to throw into the mix?