The player section of these rules gives some information about gaining XP, which are awarded for gaining treasure and killing monsters. It may seem counter-intuitive that treasure somehow makes characters more experienced, but that’s not what awarding experience for gold pieces is all about. Gold pieces are an after-the-fact measurement of how ingenious the character (player) was in getting them. The gold pieces aren’t the source of the experience, they are the measurable product of it. Solving puzzles and finding traps are all already rewarded though this “gold standard,” and shouldn’t ordinarily be the source of bonus XP. However, if the treasure from an adventure isn’t the best measurement of the effort, as in missions the players undertake without expectation of reward, the referee will have to provide an alternative source of XP. This can be done by awarding XP bonuses for finding and avoiding traps, for solving puzzles, and for turning potential enemies into allies with smart decision-making or fast talking. Overall success with a mission is another method of awarding XP when the adventure is going to be short on gold. Smart decision-making by the players is the key to awarding XP properly; avoid giving rewards for situations that were actually determined by the dice.
If you find that whatever system you’re using leads the players toward bad decisions—seeking out unnecessary combats or looking for traps to spring, for example—you might consider adjusting your system. This is true for the “official” system of awarding experience, too. Remember, the Referee is the ultimate judge of what works best for the game, and any rule can be changed to fit the gaming group.