Mushrooms

Mushrooms can be harvested for 25 gold.

Default maps
The maps that come with Wesnoth have mushrooms scattered all over. Because harvesting mushrooms costs no action points, it's a good idea to grab as many of these as you can early on.

Farming
Mushrooms can also be planted on cave terrain for a large material cost. Since this converts the hex to dirt when the mushrooms are harvested, this method of transforming material into gold is not sustainable.