Got a random encounter but can't think of something on the fly, and you don't want "just another combat encounter" ?
Roll on this handy table to see what exactly the wandering beasties are up to.
2: In pursuit of party
3: In pursuit of someone unknown to party
4: Fleeing from someone
6: Searching for something
8: Passing through
9: Carrying loot
10: Settling a dispute
Results should of course be interpreted based on the creatures. Maybe "settling a dispute" turns out to be a gelatinous cube mating ritual, while a group of hobgoblin explorers might trigger a whole sub-plot about a humanoid tribe trying to invade the dungeon.
Don't forget that reaction rolls can add to the flavour as well. The group of elves is pursuing the party, but react in a friendly manner. Maybe they are carrying a warning?
Comments welcome! If you have further ideas, possibly expanding the table to a D20, I'd love to hear them.