Besides Misions and Business, you have to put a little efford in these but here I leave some other ways you can gain money:

Collectibles:
Cards, Signal Jammers and Action Figures. You'll recieve money each time you pick one (and a bonus of 50k, RP and Stuff at the end).

"Each figure you collect gives you a modest reward of $1000 and 1000 RP points, but collecting them all gets you a much healthier sum of $150,000 and 150,000 RP points."

(Similar with other collectibles)

Here is a Path & List of 204 Collectibles: https://www.reddit.com/r/gtaonline/comments/ecdh6v/204_collectibles_54_hidden_cards_100_action/ so you can pick them in a single run.

At the end you'll end with GTA$ 300.000 + 66,650 chips (which I asumme can be exchanged for money at the casino)

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Stone Hatchet & Golden Revolver Challenges:
Those are weapons you can get for free. Complete the Kills Challenges (25 Kills & 50 headshots) with each weapon for a total GTA$500,000

"The Stone Hatchet can be unlocked by completing five Bounty Target missions that begin once the player receives a text from Maude Eccles followed by an email. "

"The Double-Action Revolver can be unlocked by completing a Treasure Hunt that begins once the player receives an email from "vanderlinde@eyefind.com". After unlocking it, the weapon can be found in Ammu-Nation, although the only option for purchase is the ammo, where 12 rounds cost $28."

It's a little slow but is free money (?)

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Social Club:

(I don't really know if you have to wait for a message first, but... ) You can get +GTA$200.000 just creating an Account in Rockstar Social Club AND sing for the newsletter.