If you have a script system in your game, you could execute a script that contains all the details about a mission. The script could spawn props at certain locations around the world, and also specify win/lose conditions. Or it could be a collection of scripts. The first script would create a trigger box at a certain location and tell it that if the player enters it to execute a second script.

Therefore, all your mission-specific logic is contained in scripts which can be easily added to your game data.