Now you can have whole hex crawl campaigns with huge maps. Or you can have a large part of your adventure being a hex crawl. For example the 5E module Tomb of Annihilation has a large hex crawl section in which the group explores the jungles of Chult. With some players and reviewers remarking that this hex crawl section can easily take too long, and become boringly repetitive. The hex map of Chult is about 50 x 60 hexes large, and the final destination isn't obvious to find. With an average travel speed of 2 hexes per day at the scale of that map, it can easily take a month or more of game time to play through this part, and many sessions of real time.

For my D&D pirate campaign, which I started with the Ghosts of Saltmarsh introductory adventure, but will now move to self-written adventures, I asked my players for things they would like to see in that adventure. And several suggestions involved exploring an island: One character had a background story where he was stranded on an island for some years, others wanted to go treasure hunting, or explore an island full of cannibals. So I decided to make an adventure with a hex crawl to explore an island. With the island being only 7 hexes broad and 12 hexes long at most, and there being just about 50 hexes to explore. And then I started thinking about what sort of hex crawl rules I will need for this. For example in Tomb of Annihilation the large hex crawl involves the probability to get lost, and the need to find food and water. For a small hex crawl, I can skip or simplify some of that, especially since inventory management isn't the most fun to play anyway.

So what rules does a hex crawl need? It basically needs two major parts: How to get from one hex to the next, and what happens in the hex you enter. For my adventure I decided that I would keep a 2 hex per day travel speed at normal pace through rough terrain. There is a small bit of road, where the group can advance at twice the speed. Every day the group needs to decide between normal and fast travel speed; fast speed adds another hex of travel that day, but increases the danger of encountering patrols of goblin cannibals. I decided that the group can only get "lost" if they want to travel in one direction and the hex 60° to the left or right of that direction is the same terrain type; in that case they need to do a DC12 survival check to not deviate from the path. This solves the situation where the group is following e.g. the coast line or the edge of a forest, in which case it wouldn't be logical for them to become lost.

And what happens in the hex they enter? The island has 4 different terrain types: Beach, Jungle, Hill, or Volcanic. In addition to the terrain type, some hexes also have special features, for example a road, a building, a cave, a lake, or a landmark. Despite every single hex covering several square miles, I decided that all special features should be immediately be obvious when entering a hex; the group shouldn't be able to "miss" a building while traveling through its hex. While the hex crawl doesn't have a "story" per se, the island has three main special features: A temple of a sea goddess, a volcanic cave with a salamander, and the central village of the goblin cannibals. The sea goddess wants the salamander gone, the salamander wants to burn down the island, and the goblin cannibals are holding a pirate crew captive for food, which the group wants to liberate to man their ship. A limited amount of such special features is necessary for the adventure to have a bit of structure, but it is up to the players how to interact with each of them. Minor special features are also sprinkled over the island, like caves that can contain various monster encounters.

When not interacting with a special feature, the content of each hex is determined when the group enters it by a random encounter roll. I decided to use d20 for that, with low rolls resulting in no encounter, medium rolls resulting in the group receiving a clue (e.g. footprints) to some special feature or a terrain encounter or a random combat encounter, while high rolls result in an encounter with a patrol of goblin cannibals. That linear scale allows for simple modifiers, e.g. +4 on the roll if traveling at fast speed, leading to a lower chance of nothing happening, and a higher chance of the specific combat encounter with the cannibals.

And that is all the rules I need here. The main work is to create terrain encounter tables and random combat encounter tables for each of the different terrain types. But once I have that, I can play probably at least 2 sessions with the group exploring the island sandbox style.