There's a lot of different ways, you can use a farming/herblore combo. I'm 76 farming and 75 herblore and I made around 30m from it sence lvl 30, u can woodcut yews after 60 woodcutting , you could fish lobsters, thieve, u can basicaly make money from everything except firemaking.
One of the fastest, at least on f2p is soft clay. There is always a huge requirement from people to lazy to get their own. From the Varrock-West bank, weild a pickaxe and 4 buckets. Head west just outside the city wall then follow the wall south to the mine. Mine as much clay as you can carry, then head east to the Varrock southern entrance then North to the foundtain. fill the buckets then mix with clay to make soft clay, and repeat. (you could use just 1 bucket but this takes ages). Then its a quick run west back to the bank and start again.
Depending on how busy the mine is, 1 trip takes about 3 or 4 minutes and at 208gp each, you can earn 5k per 1 trip... about 75k/hour.