I devised a 5e random hex crawl chart that tracks weather, monsters, survival checks, and all sorts of stuff, all on a d12 table. I already shared my hills encounter chart; here’s one that I used in my recent Underdark campaign, in case you want to try a cave crawl (spelunk?).
As a reminder, you can make your own chart for any terrain; for each entry, keep the part in bold and write a new location-appropriate encounter.
1: Plot advancing creature: In my game, this slot is filled by an evil cherub messenger of a sinister angel.
2: Intelligent creature: A drow party traveling with recently-captured slaves, in a spiked, spider-drawn cart, towards the nearest drow trading city. The drow will attack and enslave weak groups, or bargain with strong groups.
3: Unintelligent creature: A wandering behir. This monster is among the toughest on this particular random chart, and in my game, it nearly wiped out the PCs. The nearly-dead sorcerer, in a hail mary pass, managed to Polymorph the behir into a chicken. In 5e, Polymorph is a Concentration spell that ends when the creature dies. That meant that the PCs had a limited time dispose of a weaponized chicken, which they did to good effect, taking out a bunch of drow and trolls along the way.
4: Ambush creature: Green slime often surprises the victim, as do umber hulks. What about the two together? An agonized umber hulk, slowly being dissolved by green slime, lurks around a corner, trying to scrape off the slime but just moving it around its body. If it hears PCs approaching, it will attack with a suicidal fervor fueled by rage and pain. In combat, its attacks might infect the PCs.
5: Beneficial creature: Scouts for an army of deep gnomes, methodically mapping the tunnels. They’ll lead friendly PCs to High Commander Vilkrieg, commander of the Loose Gemstones Free Army, who’s looking for a path that will let him surprise-attack the local drow settlement.
6: Weather: Torches flicker blue. Those with Detect Magic sense eldritch weirdness. The long straight tunnels of the underdark give way to dungeon-style branching corridors and wooden doors, some in the process of budding, as if the dungeon were growing like a plant. The party has discovered a pocket of the chaotic, half-mindless gas that, coral-like, leaves behind the strange dungeons of the Mythic Underworld.
7. Lair: Drow checkpoint: The tunnel is guarded by one elite and two regular warriors; another elite warrior, two regular warriors, and a wizard are relaxing in a fortified suite of rooms built into the side of the tunnel. There are two portcullises that can be dropped across the tunnel. The drow try to trap intruders between the portcullises and use Darkness and missile attacks to confound them.
8. Survival Check or Hazard: Sinkhole. Dex save or fall through a weak floor into an east-west purple worm tube. Following the tube eastward will lead through a forgotten dwarven tomb; westward will lead to the purple worm.
9. Path Choice: The tunnel branches. From around the curve of the left side, you hear an echoing argument about directions in Elvish. The noise is from a kenku in a cage; bones litter the floor. If it sees the PCs, the kenku will declare, in elvish, “It’s a trap!” Intelligent giant spiders descend and attack the PCs, declaring, “Our Kenku is developing a sense of humor. He must be punished.”
10. Beneficial location: Old dwarf palace lit by 2d10 sunstones (worth 1d6x100 GP each). The stones glow. Any stone worth 600 GP glows brightly enough to act as sunlight for all purposes, frying vampires, granting disadvantage to drow, and damaging drow equipment.
11. Ruin: A vast chasm, dotted with lights below. The chasm is a mile deep. At the bottom, terrified goblins tend bonfires amid the ruins of an ancient city. When the fires go out, they’re preyed upon by an underdark monster who uses illusion to disguise as one of the goblins – but the illusion only fools darkvision, not natural light.
12. Tracks: Green slime footprints lead to encounter 4 (and let you potentially surprise the umber hulk).