Old-school D&D suggested rolling 1d6 every 30 minutes of exploration time, and on a 1, a random encounter occurs. You can also make extra rolls if the heroes make a bunch of noise or otherwise call attention to themselves. But there’s a better way.

Goblin Punch has an interesting post on random encounters that introduces an “underclock”. This has a certain number of points that get reduced by a 1d6 every so often. When it’s down to zero, you get a random encounter. It’s slightly more complicated than that in order to introduce hints that a random monster is lurking, but that’s the heart of it.

I think this is a prime opportunity to use my dice timer mechanism. When you first roll (every 20 minutes of exploring), you roll a d20. Next time, it’s a d12; then d10, d8, d6, d4, & d3.

Results:

1. A direct encounter
2. Weird noises nearby (roll for the creature)
3. Droppings or the remains of a meal
4. Tracks or the remains of a sleeping nest

I say that martial characters have a monster lore skill they can check to try to identify physical evidence (harder for a meal or nest).

If there’s no encounter even from the d3, it happens the time after that. This guarantees an encounter in no more than 8 opportunities, while–using the old-school method–it was very likely after 6 opportunities, but never guaranteed. (If you prefer that an encounter never be certain, you can just keep rolling the d3.)

In a wilderness, decide on a specific location for the encounter to occur, such as a ravine, pond, stream, slope, small cave, minor ruins, small waterfall, rock outcropping, etc.

Sounds and smells of combat may attract certain monsters but may repel others, so reset the die to the d20 but consider which creatures might be drawn to the area. After all, if orcs hear fighting in their lair, they’ll surely come running with weapons.