Even though it is a challenge in itself, now comes the actual RuneScape gold bummer - Wrath Altar is obtained via Myth's Guild by climbing down Mythic Statue and then running through caves full of chromatic dragons. It's worth it. On a negative - altar can not be obtained via Abyss and nor the output could be doubled. You'll require Fletching skill level 70, yew logs (Woodcutting ability level 60) to cut the yew longbow (u) out of these, then bowstring (Crafting skill level 10) to string the bow. Both cutting on the bow and stringing it return Fletching experience.
HERBLORE MAKING UNFINISHED RANARR POTIONS. Ability Herblore to 25 in order clean Grimy Ranarr Weed - it drops out of Combat ability level 13 Chaos druids (that is a great cross gold creating opportunity since Chaos druids could be farmed for many precious items) and continue doing so until attaining level 30 in Herblore ability when you may unlock skill to craft unfinished Ranarr potions. Afterward, as usual - put it on Grand Exchange and expect for huge profits.
SMITHING. Although smelting bars is closely interlocked with Old school rs gold Mining, you don't have to mine ore yourself - requirements for efficient smelting bar enough people from doing it, so it is rewarding to buy a good deal of raw stuff by the Grand Exchange, smelt it, and then post again as a refined goods.
