Someone on the OSR Discord server asked me to write this up in a blog post, so I thought I would talk a little bit about terrain for hex maps.
When I am creating hex maps for overland travel, I typically use 6-8 types of terrain so that I can assign them to a randomizer and have the PCs roll whenever I'm not sure what a given terrain type will be if I don't already know. The eight main types of terrain I use are:
I sort the terrain into three categories based on ease of traversing it:
Easy: Blight, Desert, Plains, Water (with watercraft)
Difficult: Forest, Hills, Wetland
Impassable: Mountains, Water (without watercraft)
Easy terrain allows PCs to move through it at their normal movement (10km per 4 hrs of travel per the Procedure for Exploring the Wilderness Redux). While following a path in easy terrain, you cannot get lost.
Difficult terrain has PCs move through at 1/2 the normal rate (two travel actions must be taken to cross it). Paths across difficult terrain double your movement: It costs one travel action to move across the hex).
Impassable terrain cannot be traversed unless the PCs find a path across it, and they can only traverse it in the direction the path does. Paths across water can represent significant shallows or fords or small island chains close enough to swim from island to island.
Within these terrain types, I aim for a certain level of variation based on what makes sense for a given area. In a setting based on Scandinavia (all the rage right now), a desert will be an alvar pavement, a forest will be mostly coniferous, and wetlands will be bogs. In a setting based on Nigeria, wetlands will be flood forests, a forest will be an acacia / peacock flower / long grass mosaic, and hills will be a classic West African highland rise. I don't bother to mechanise this fine a set of details.
I find this tends to incentivise looking for paths, especially when PCs want to cross formidable natural barriers like mountains or lakes.