This is just basically going around killing things and picking up the stuff they drop. It is a good way to make money if your lv3 or lv138 depending on what you’re killing.
Monsters you can kill are
Chickens: Can be found north of Lumbridge at the farms or Southeast of Falador beside the cows. Chickens only have 3 hitpoints which makes them very easy to kill and their feathers sell for around 5-10 gp. Since feathers are stackable there is no need to make trips to banks.
Recommended Level: 3 and up
Cows: Can be found close to anywhere there are chickens. Most popular are north of Lumbridge and Southeast of Falador. These places are usually packed with people as cows are good training as well. Cow hides sell for about 140gp at the GE.
Recommended Level: 10 and up
Minotaurs: These are found in the Stronghold of Security 1st floor. They are not the best money earners but are very good training and usually drop sellable things like iron arrows, uncut gems and noted rune essences.
Recommended Level: 20-30 and up
Hill Giants: Main source of money for a lot of people and is a highly popular way to make money. Can only be found in Edgeville’s Dungeon and are killed mainly for their big bones which sell for around 450gp each
Recommended Level: At least around 30 but good accounts by lv45 can easily own them.
Green Dragons: Very good way to make money but being high level is a must as they are only found in the Wilderness most common places being level 13 near the dark knight fortress and level 20-24 near the shore to the east. They are mainly hunted for their green dragonhide sold at around 1800gp and dragon bones at around 3k.
Recommended Level: 60 and up with good armour and anti fire shield. Watch out for other things
Lesser Demons: Still very popular even through best drop is rune med helm. Why, because it has weak defence and max hit on you are normally 6-7. Also the place you fight them at (usually Karamja Volcano) has like 5 safe spots to mage or range them at.
Recommended Level: 70 and up depending if you range, mage, or melee it.