I've mostly talked about random event tricks in a settlement campaign. However, these are pretty trivial elements that could be replaced with other event generation systems. For example, a couple people have mentioned the events rolls from Oriental Adventures. The real crucial element needed to make a settlement campaign work is to make changes to the way experience is awarded.
The default is to award experience points for monsters slain and treasure reclaimed. There's not much treasure in a settlement campaign, unless it's a bandit settlement established along a trade route. And after clearing the settlement area, monsters show up pretty randomly; you can't rely on standard combat as the driving force in the campaign. You could, however, compare the population and wealth of a settlement to the monsters and treasure in standard encounters.
The primary goal of a settlement is to maintain a stable population and, if possible, grow; you can compare the total hit dice of NPC settlers before the start of a season to the hit dice at the end of the season, to see if there is any growth. Each PC receives 100 xp per total NPC HD if the settlement grows; otherwise, they receive 10 xp per HD. (I recommend using the total, rather than the difference, just to make it easy.)
The secondary goal of a settlement is to generate extra wealth. A settlement can only generate wealth in Good or Very Good seasons; it is assumed that all wealth in Average or Bad seasons is either consumed by the settlement or traded for necessities, and only a surplus matters, since it allows growth beyond the current level. To reuse existing rules and make things simple, treat the surplus generated by one project in one Good season as one piece of jewelry found in a treasure hoard. Thus, if one player is prospecting, a Good season means that player retrieved the equivalent of one piece of jewelry. If the player has two assistants, that's equivalent to three pieces of jewelry in a Good season. In a Very Good season, roll 1d6 per person involved in the project. The total value is determined as you would determine the value of jewelry in a hoard. In Monsters & Treasure, the suggested rule is to roll percentile dice to determine if a piece of jewelry is Low value (3d6 x 100 gp,) Average value (1d6 x 1000 gp,) or High value (1d10 x 1000 gp.)
Every project that can produce trade items is checked for wealth produced; services (road construction, fences, patrols) don't produce wealth directly. Some projects can't produce wealth in some seasons (crops,) or can't produce wealth until other resources have been acquired (wool requires sheep.)
There's probably more that could be done here, such as investing any surplus into other industries to get a bonus to wealth produced, or placed in reserve to add a bonus to a future roll.