A specific, planned encounter for a hex does not have to be especially complicated. It can be as simple as a quick meeting with an explorer who can sell the PCs some necessary supplies or the discovery of a monster lair that hints at a greater threat. A good rule when determining the number of planned encounters to prepare is to have at least one for each character in the party. That way, you can tailor encounters to allow each character to take the spotlight without having to populate every single hex on the map one by one.
After creating these encounters, choose a hex on the map and note that an encounter occurs there. When the party draws closer to a hex with a planned encounter, foreshadow it with appropriate details. For example, if you plan to have the party discover a battle between two armies, the nearby hexes should contain signs of an army’s passage—old cooking fires, piles of refuse, and even the graves of soldiers who fell to illness along the way give your players clues about the impending encounter.
A few encounter sites are landmarks immediately obvious or visible with just a little bit of looking or scouting. A PC who enters the hex automatically discovers the landmark. If a PC in an adjacent hex spends an hour studying the landmark’s hex and succeeds at a DC 10 Survival check, he discovers the landmark. When the PCs discover a landmark, note it on the landmark’s hex.
Many encounter sites remain undiscovered unless the PCs decide to explore a hex rather than just travel through it. By exploring the hex, the PCs discover the site automatically. Some sites are hidden, requiring the PCs to make an appropriate skill check as they explore. The skill and its DC depend on the nature of the site.