You can go to Supersonic to get split around, merging and gathering exp while grinding coins. Me, myself I go to Battle Royale NA and not go for mass only but also go for the little coins that spawn across the map. After a few rounds, if I win, I then go to Agf.io. Since you split around crazy fast and can travel around the map within 30 seconds, grinding is easy. The coins are worth 300 each (or 400 each my memory sucks sorry :c ) and within a few hours I can grind 100k-200k coins. I promise you if you have the time, you can get rich if you try hard enough.
(btw if u go into Giant and buy 300 bots or 100 mass bots, you can get a lot of mass and farm the gold blocks. so just buy the bots :] )