After completing a task, there is always the expectation of a reward by the players. Even though material rewards are common in most situations, meeting the players expectation is all that is needed to reward them for their adventures.
For example: The party goes adventuring in a very poor area of the country, full of dangers but very poor in spoils. While they may be fighting a lot, there will be very little in terms of treasure for them.
Under those situations, the players need to be rewarded with something other than money or loot. That reward has to possess some inherent value to the players. Enough of it to compensate them for their efforts.
In the example above: The locals, while poor, could be rich in their gratitude to the party. Organizing events in their honor, singing songs about their exploits, even naming their children after the characters.
These gifts by the local population will have value to the players. Having their characters cherished and loved will give the players a sense of accomplishment further enticing the party to adventure.
Meeting expectations is all that is needed to reward the players. That reward can be material or anything else the party may value.