The adventurers left the dungeon to recoup, and now they are back. But what happened while they were gone? You can use the following encounter change tables to shake things up a bit.
You can use this for encounters that the characters had, but left unfinished.
1: Encounter remains unchanged
2: Creatures have moved to another nearby area
3: Creatures have moved to faraway area
4: Creatures have allied with other creatures
5: Creatures have gained reinforcements.
6: Creatures have improved defenses.
You could easily elaborate far more on a table like this, and I may do a longer version later on. For now, this will do fine for a quickie.
Note, this table has been replaced with an updated version here