1. Mining(requirements: any pickaxe):
Mining is one of the skills that take a very long time to level up. If you want to make some decent money try to get your mining up to level 40 so you can mine gold ores. While you’re training your mining store all your ores into the bank. When you reach level 40, mine gold until you have like 100 ores and then withdraw all your ores and go to grand exchange. The 100 gold ores alone will give you 40k and if you add the rest of the ores you might go over 100k gold. If you already have your mining level 40 or more just mine gold ores and sell them.
2. Big Bones(requirements: Brass Key, be able to defeat a level 28):
The big bones are very searched among Runescape players who want to train their prayer skill. To take advantage of this fact and make some money get yourself a brass key(Edgeville Dungeon), go west from Varrock until you find a small house, use the brass key and start killing Hill Giants. They are level 28 and they always drop Big Bones. 100 Big Bones are like 40k so it is a good way of making money. Hill Giants may also drop runes and other valuable items.
3. Woodcutting(requirements: woodcutting level 30+):
Many Runescape players buy logs through Grand Exchange to train their fire making skill or their fletching. The most searched logs among non-members are Yew (level 60) and Willow (level 30). Woodcutting is a very easy to train skill and you can make good money out of it. It might take a while to reach level 60 but it will worth it.
4. Fist of Guthix(requirements: high combat level, good armor, good weapons):
Fist of Guthix is one of Runescape’s many minigames. Basically you have to gather as many points as you can before your opponent finds you and kills you and then you trade places. The one with the most points wins the game and some Tokens with which he can buy items. The items you can buy in this minigame are pretty valuable and you can make good money out of it.