Even though I have run many published campaigns in my GM life, my preferred campaign style is that of an open-world sandbox in a long-term campaign. Part of any good sandbox campaign is the use of a good map. By good I don't mean the quality of the art (even though that is great to have), but rather how useful the map is. The best type of map for a sandbox campaign is a hex map.
Whether it was the Isle of Dread, Mystara, or the famous "Darlene Map" of Greyhawk, many early D&D modules and settings utilized hex maps. One of my favorite aspects of the old AD&D 1e Forgotten Realms "grey" box set was the hex overlay that you could put on top of the supplied maps. Hex maps are a GM's best friend due to their ease of use. You can quickly gauge travel distances and -times, as well as terrain features and adventure locations.
I use hex maps extensively in my Old-School Essentials campaign setting. My preferred style is a map where each hex shows the predominant terrain type. The BECMI Mystara maps are great examples of this style of hex map. Below is a copy of one of my OSE hex maps (please excuse the poor drawing skills).
Here you can easily see the dominant terrain features of each hex, be that a settlement, a river, forests, etc. In this map, each hex = 6 miles (in future I will be scaling that down to 3 miles). This basic black-and-white map serves as the basis for the campaign area. Some might think that this is a very small campaign area, but it represents a region that is just over 3,000 square miles. Once I have this first step done, I create a color version for the players (see below).
Color coding each hex and the major water features allows the players (and Referee) to quickly and easily identify the terrain types. It's a minor color addition, but makes a huge difference versus the simple B&W map from above. It also further reinforces the terrain types.
With that map done, I will then create a further version of the map just for the Referee (see below).
This version of the map is not color-coded by terrain type, but rather by encounter/threat zone. Certain hexes also have static numbered adventure locations not present on the player's map. This particular map contains 20 static locations. The encounter/threat zones (A - E) have a corresponding random encounter table. Whenever the party is in a hex, I roll to see if they have a random encounter on a single d6. The odds of a random encounter increase as the threat level of the hex increases. The 2d6 random encounter table is also built to simulate the danger present in each hex. In a Zone A hex, for example, PCs are more likely to encounter peaceful or mundane people and animals than in a Zone D hex. Every zone has different encounter tables for each type of terrain.
Having this final Referee hex map is vital for a sandbox campaign. Not only does it give me a great overview of terrain types, travel, and hidden adventure locations, it allows me to quickly determine on-the-fly random encounters. The great thing about Old-School Essentials is that the monster stat blocks are quick and easy to run. Even though I have done this for games like D&D 5e and Pathfinder 2e in the past as well, but the way monster stat blocks are presented in those games makes it a bit of a slower process.
For OSE I never pre-roll random encounters, since I never know which way the PCs will end up wandering. It is a sandbox campaign after all, where the players are in full control and have absolute agency about what their characters do and where they go. Unlike the linear "story path" adventures from D&D 5e or Pathfinder 2e, in my sandbox campaigns there is no predefined set direction the characters must follow. Therefore, having a flexible and easy-to-use map is an absolute necessity.
To simplify the overland travel in OSE, I gave each hex a travel point cost (plains = 1, forest/hills = 2, swamps = 3, mountains = 4) with roads cutting the travel point cost of a hex in half. I then took the base movement rates of Old-School Essentials and converted them into daily travel points. Since there are four levels of encumbrance in OSE, each level determines the available travel points per day. An unencumbered PC with a base movement rate of 40' would have 4 travel points to spend per day. Each encumbrance level reduces the travel points by 1. My players have really liked this system and I run OSE for veteran adult players and my children's kids group (new players ages 10+). The new Dolmenwood system by Necrotic Gnome (creators of Old-School Essentials) also uses a travel points mechanic for overland travel, which I really like (you can read up on it in their free preview PDF here).
In future I will write an article detailing how I create random encounter tables for Old-School Essentials (and other OSR games), but I hope this quick overview of my hex map design process will serve as inspiration for you to develop your own hex map.
I have a personal goal to encourage as many RPG players as possible to give long-term, open-world sandbox campaigns a try and will be presenting more easy-to-use tools in future to help this process.