Here are some specifics on how I've been doing them in The Lost City, the jungle-and-ruins section of my DF world.
I decided on a few principles, learned from my wandering monsters in Felltower and then my attempts to merge non-monster encounters with monster encounters (ala Mirror of the Fire Demon) in the Cold Fens.
In the Jungle
Roll every 4 hours, once overnight if camped. 12- is an encounter.
1-Obstacle (Appropriate skill roll by the point man - usually Navigation to avoid it, Climbing to go over it, Swimming to wade it, etc. and a 30 minute delay. Failure doubles the time needed.)
2-Major Obstacle (As above but 1 hour delay.)
3-Nuisance Bugs (1d-4 FP, plus HT roll to avoid Jungle Rot at -2 to +3 based on roll.)
4-Nuisance Bugs (same.)
5-Nuisance animals (rats, army ants, fire ants, spiders, snakes, etc. cause 1 hour delay.)
Obstacles are anything from washed-out sections of trail, fallen trees, flooded areas, collapsed root tangles that once acted as a path, etc.
The "nuisance bugs" roll elicits the most groans. The rule is really poorly written - I'm trying to say that :
- you lose 1d-4 FP (so, 0-2) that must be cured by rest.
- everyone gets bitten by bugs when this happens, causing a HT roll vs. Jungle Rot at HT+4, minus your die roll. Roll a 6, take 2 FP and roll at HT-2. That's probably how I should have explained it.
- It doesn't say it, but I allow substituting HT-based Survival (Jungle) to avoid this.
For Lethal Encounters, I just have a list for the jungle, and I know what's in the area or that I want to use to have a proper jungle flavor. So it's pre-decided monsters, basically, that I roll to see if they appear. These have been:
- rhino beetles
- poisonous ants
- giant snake
- an octopus blossom (a killer plant
- a single extremely venomous normal spider (roll randomly to see whose bare skin gets bit, and go from there.)
and probably others I'm forgetting.
For the lethal encounters, I have any watchers/scouts roll Perception to see if they spot it. The better they win the contest by, the further out they see/hear/smell it. This has the added benefit that low-IQ low-PER skeletons, magical servants, and knights with vision-narrowing helmets might be handy in a fight but aren't significantly useful guards.
In the City
Random encounters happen every hour in the city on a 9- during the day, 12- at night. Add 3 if the party is being especially loud or engages in combat.
Encounter is either a close-by resident coming to investigate (or heading home) OR roll on the table.
1- 1d6+2 [redacted] from area P in the Northern part of the city, or 2d6 [redacted] in the South.
2- [redacted] from S (66%) or R (33%), double numbers at Night.
3- Giant snake (50%) or 1d6-3 Giant Bombardier Beetles (50%)
4- Giant [redacted] (50%) or 2d Giant Ants with 1d6-2 soldiers (50%) (no ants at night)
5- Giant Rhino Beetle (once!) or 1d [redacted] (will spy on PCs unless they look vulnerable).
6- (Night) [redacted] (Day) Harpy and dire vulture fly-by.
Distance: 1-2 close, 3-4 medium, 5-6 long.
The distances are vague, because it lets me choose by location and monster what that means. Even a "close" fly-by is going to be further than "long" for a giant snake encountered inside of a ruined building.
These are more specific than the jungle because I have a specific set of creatures in the ruined city. I'm basically just using a random roll to help me pick from the critters and residents that might be out and about if there isn't something hostile and mobile close by.
And yeah, there are technically spoilers in that list even with the redacted parts. That's okay - my players have encountered some of those critters already, and I only roll if I don't have something to hand.
Some more lessons I learned:
I Prefer Lists over Tables
Especially during outdoor travel, I found I prefer to pick from a list instead of rolling. Sometimes I roll something that just doesn't fit the circumstances well. Sometimes the players have been acting in a way that really should attract certain creatures. And as people travel, they go from the likely spots of certain creatures into the likely spots of others. I found I'd make a list, roll, and then cross stuff out (leaving automatic re-rolls for later) or I'd just re-roll when something that didn't fit came up. It was annoying.
So I went with just a list of likely encounters, and pull from that. Walking around with a bunch of zombies in the jungle? More likely you get flying bugs, scavengers, carrion-eating beetles, etc. Being stealthy and quiet but always take the exact same path? Ambush hunters are likely. Making a lot of noise? That might scare off the low-IQ animals but really make those nearly-sapient ape-men interested or attract that group of bandits. Etc.
Wandering Damage is Good
The travel delays, the obstacles, the nuisance bugs - those are really a lot of fun. I roll them, they get the job done without actual play time delays, and we move on. They add a lot of color, have significant impact on the game, but not significant drain on the fun. They also drive a lot of player experimentation and reaction when trying to deal with them. I highly recommend them.
How about my megadungeon?
I'm still debating how I want to implement these ideas in my megadungeon. I can't just stick in delays, but I can stick in more nuisance animals, bugs or spiders, strange smells or spills, residual spell effects, new traps (a number of residents set traps), etc. I think I will try to do that.