Since there are various guard can be found around the Runescape, some or most of them can be killed and you can make more RS gold by looting their drops.
IMG Source: quickmeme
Killing guards can be easy as pie and can give you more RS gold
Guards can be easily found in various locations especially in Gielinor which six of them are located at the Varrock Palace. When the guards are killed, they will drop grapes which are used for making wines and this can be use for RS gold sell.
Source: wikia
To start killing guards, teleport to Varrock using the lodestone and go north towards the palace. Around the palace you will see some guards. Start attacking them using whatever skills you want (they are weak against air spells). Each guard will take at least less than 60 seconds to respawn, which is good enough to collect more grapes. Once your inventory is full of grapes, you can start selling them at the Grand Exchange to make more RS gold.
Collecting Cowhides to make more Rs gold
The Cows in Burthorpe are level 2 with 25 health points which always drops cowhides. The cowhides can be brought to a tanner to turn them into soft leather for free or hard leather which is cost 3 RS gold. You can find the tanner at the Al-Kharid, the Ranging Guild, Varrock, Canifis, and the Crafting Guild. The cowhide can also be used in Creature Creation in order to create Unicows.
The cow pen can be found south of the lodestone in Burthorpe. Since cows can respawn instantly, you can kill them as much as you can. For some low level players, they can use this to train their Combat skill. Once you killed a cow, loot the cowhide. Fill up your inventory with cowhides. Once you’re done, you can now start selling them at the Grand Exchange for more Rs gold. OR you can convert them into soft leather for free.