My advice is not to spend millions to powertrain to a cape that doesn't give you any benefits. That rules out firemaking, farming, crafting, slayer, herblore, smithing, cooking, and construction. Possibly fletching as well. Those skills all stop giving any tangible benefits well before 99, so it's not worth spending a lot on them just for the cape. You can train them, but after they max out you shouldn't concern yourself too much about leveling up. Aside from that, try getting a few skills to the high 80's and see which you could stand for the longest.