My favorite! My personal preference is Crafting. You can slaughter cows in Lumbridge, take their cowhides and turn them into leather (or hard leather) when your crafting level is greater (28+). You can take 28 cowhides (or 84) and create leather or hard leather using 28 coins. Find a needle and thread and begin making. For f2p (free to play) people the best thing that you can craft using leather is a studded leather body.
Have you ever been at a random event, gotten some kind of gem and said, "Whatever... I will just deposit it into my bank and wait until I'm in need of it"? Now you've got the gem. Crafting can turn precious stones and gold into pieces of jewelry. A crafting lvl 22 is required to create the simplest jewelry piece that includes a gemstone. You will require a casting lvl 22 to make jewelry with gemstones. This is how you "use" your furnace to make the jewelry you want. It's easy! It's an excellent skill and if you master it correctly, it will provide you with a great deal of wealth.