Sinister Chests

All you need for this method is a little cash to buy some keys and sometime to open the chests

The sinister chest, located in the Yanille Agility dungeon, always rewards nine herbs when opened: three grimy ranarr, two grimy harralander, and one each of grimy torstol, grimy irit, grimy avantoe and grimy kwuarm. Opening the chest requires a sinister key, and the key is used up when the chest is opened. Depending on the price difference between the keys and the herbs, it can be very profitable to buy sinister keys, open the chest, and sell the resulting herbs. The price of sinister keys jumps around a lot, but it's not uncommon to buy them instantly for far below the market price. Start out by summoning your best Beast of Burden -- a war tortoise or pack yak is recommended. Wear a slashing weapon of any kind and your TokKul-Zo, and withdraw two law runes, along with 5 sinister keys (war tortoise) or 6 sinister keys (pack yak). Teleport to the Watchtower, go down the ladder, and run east to the dungeon entrance. Slash the web, go down the stairs, and across the agility ledge. Enter the obstacle pipe immediately next to you and open the chest at the end of the room. You will be poisoned, but this only hits for about 15 damage and can safely be ignored. Keep opening the chest (it takes about 8 seconds each), storing the herbs in your Beast of Burden as needed. Depending on the price of the grimy harralander, it may be worthwhile to drop them so you can use more keys per load. When you've used all of your keys, teleport out using your TokKul-Zo, bank the herbs and repeat. When using the war tortoise pouch, you can open about 135 chests per hour. When using a pack yak, you can open about 150. Sell your herbs on the Grand Exchange when finished.

Link to a video on this method

Happy money making everyone