Making oak planks at the sawmill has long been a decent way to make money. Each plank requires one oak log and 250 coins. The fastest way to get to the sawmill is through the balloon transport system, which requires completion of Enlightened Journey. Once you've unlocked the Varrock route, requiring 40 Firemaking, you can fly to the Varrock balloon, just steps away from the sawmill, for one willow log.