Big Bones!
Go to Edgeville dungeon n find a brass key. Go back out and go to a tiny little shack in between Varrock and Edgeville. Use the key to open the door. Kill the level 28 giants. You usually get big bones and another item. Keep all the big bones and when you get a full inventory take them to the bank then go back and repeat. This gets your combat level up as well although if you are lower than level 28 you should bring food. You can sell these big bones for around 300gp each.
Cowhide!
This is an easy way of making money. First of all, go to the cow farm in Lumbridge. Pick up any cowhides on the ground, and kill cows for them. Keep doing this until you have a full inventory of cowhides, then go to the bank and store them. Keep doing this until you have lots! Then get 10gp, you can get this by killing a goblin or a man, and go into Al Kharid. Go onto a popular world such as world 1 and sell the cowhides for around 100gp each.
Lobsters!
Get your fishing and cooking level up to 40. Take 60gp and a lobster cage and go to Karamja. Once there, go and fish lobsters until you have a full inventory. Then go to the range in port sarim and cook them. Go back to Draynor bank and store the lobsters. Take another 60gp out and repeat it until you have about 1000 lobsters. Sell them for 200 - 250gp each. You'll be rich in no time
luff ya babe