Playing in a sandbox campaign means the GM needs to be fairly quick on his feet with improvising situations since there is no direct point to point exploration. Players can wander where ever they please and create the adventure organically without the heavy handed scripts that fill the beginning of most adventure modules. For a good sandbox adventure to succeed the GM needs to understand the ecology of his area to give the adventure and place a sense of depth. That life continues on in this area even if the players never set foot in the hex.
The following is an example of how to build a wilderness hex and by no means the only way to go about it. I do it differently than Rob (Bat in the Attic) and Dwayne (Gamers Closet) so find what works best for you and go with it. I'm not ashamed to say I've stolen elements from most of the GMs I've gamed with and incorporated those elements that would enhance my own style.
A campaign map will have hundreds of hexes with the basic information, a symbol representing water, forest, mountains, hills or plains. A dot for a village or city. A square for a castle or fort. What I am focusing on is all that space between the dots. The places adventurers must travel to find the lost tomb of the dead god of two many vowels in a row or that tower with obligatory orcs and femme fatale princess in need of saving.
Now you have your beautifully rendered map in front of you and you have to decide how large the hexes are. Mine are 12 miles across. Then you have to decide how many miles a party can travel. The travel times I use are the ones I get from Rob Conley. These distances are based on an 8 hour travel day and weather penalties are not included. Light forest and plains is 20 miles, heavy woods 10-15 miles, jungle/swamp is 5-10 miles. A good road a party can increase their distance up to 50% or a trail can decrease the difficult of the terrain by one.
Say your adventuring party has discovered the location of the goblin stronghold. The stronghold is located four hexes into the wilderness. The party will be traveling on a road for one hex, lightly wooded area for two hexes and the stronghold is located in a heavily wooded hex. The party will reach the middle of the 2nd hex after the first day of travel. So let's take a look.
Wilderness hex 2301 is a section of the Aberdeen Forest that borders the heavily forested hills of the Cranax Wayland to the north. There are no roads or heavily travelled trails here. This area is inhabited by typical forest animals including a giant grizzly bear, a small tribe of bugbears has moved into a pair of shallow caves in the northern hills and transient population of goblins from the stronghold. On occasion woodsmen from the villages of Orton and Green Tree can be found in the southern area of the hex. Then there is the Ghost of Aberdeen, a knight who has roamed the forest for years.
A brief description of the listed inhabitants above:
Giant Grizzly Bear: This beast stands 15' tall and weights over 1300 pounds. He usually stays by the streams to hunt and sleeps in the caves to the north of the hex. There are two broken arrows in his right side from a recent battle with the bugbears. The pelt will bring in a hefty price at market.
Bugbears: Twelve bugbears moved into the caves hoping to capture some of the goblins for slaves and take their treasure. Two of the bugbears were killed by the giant grizzly bear in the area. The bugbears are led by a dark shaman. He always travels with at least four body guards.
Goblins: Twenty to forty goblins travel though this hex two to four times a year on their way to a raiding camp. When they do a return trip their numbers are reduced by 20% to 50% carrying 100gp-1000gp worth of items, food supplies and coinage. When the goblins return to the raiding camp their numbers will be replenished.
Woodsmen: Paul is from the village of Green Tree and Jerrin and Lloyd are from Orton. They know each other well and are friends. They trap and fish in the streams. They know the area well. They know the location of the Well of Whispers, they've all seen the giant bear and the Ghost of Aberdeen, and they know to stay clear of the goblins when they come marching through. They don't know about the bugbears yet. All of them are competent combatants, but only do so in self defense.
The Ghost of Aberdeen: He is not a ghost, but a nameless knight who has patrolled the forest on his massive mount for over twenty years. He patrols the hexes surrounding this hex, but his main home is by the Well of Whispers. He does not speak and will not engage the players in any way. If attacked he will defend himself, but he will not kill. In combat he uses his double crossbow from a distance and a long sword and shield when engaged in close combat. His shield is battered and in poor condition as is most of his equipment.
A random encounter table would look like this;
1-5 d4 Wildlife
6 Giant Grizzly Bear
7-8 d4 Bugbear Patrol
9 d4+4 Bugbear Patrol + Dark Shaman
10-13 d20+20 Goblin Raiding Party
14-17 d20+20-25% Returning Goblin Raiding Party
18-19 d3 Woodmen
20 The Ghost of Aberdeen
The four stone long houses (the ruins) in the central part of the hex have nothing of value in them or around them. A person well versed in history would recognize the structures as those the North Men used to construct and in the center is a well. At night hundreds of voices can be heard coming from the depths of the well. Wells were holy places to the North Men. They placed their sacrifices in these wells and also offered their dead.
Now an enterprising GM could construct a cool little dungeon adventure here or keep it as an unexplained mystery. For this example this hex is to be travelled through, but this is a sandbox campaign so a GM may want to have some idea of what the players may find if they decide to explore. In just under 600 words I've got a pretty good picture of what is going on in this hex and how it influences the areas that surrounds it so if the players decide to explore, the GM has enough details to fill in the bigger picture.
Be flexible. Be prepared. The goal of the players in this example is getting to the goblin stronghold, but the journey there is just as important.