You could try the
Galactic Milieu's free-as-in-beer interface, which uses
Crossfire-RPG servers.
The Milieu uses existing free open-source online multiplayer games, so the part about being an actual game is already built in, Crossfire-RPG for example is an enduring classic, it has been around for decades and likely will be around for decades to come; it has players who have been avid players for decades and developers who still work on it.
Thus it is a proven game for gaming's sake, not some newfangled made-up thing invented purely to find an excuse for minting billions of tokens to pump and dump.
Fire up a Crossfire-RPG server and tell it to connect to Server1.Knotwork.net
Free-as-in-beer means no cost to play; so far that has been sustainable despite giving out free of charge a character and some gear for it and even a bit of currency to get started with, though of course if people start taking advantage by creating a character, dropping its gear, deleting the character, starting another drop its gear with the previous one's and so on all day just to pile up oodles of free gear to sell to shops eventually even this rabbithole into the Milieu could someday end up having to involve a startup cost just to prevent such character-spamming.
The main interface to crypto in the Crossfire-RPG servers is AMBerium, the value on spot markets of which you can look up on
Stellar as the asset CCAMB (CrossCiv AMBerium) or on
HORIZON as the asset MGOLD (MUD Gold) since we equate Crossfire-RPG's goldpieces to CoffeeMUD's goldpieces so one AMBerium is fifty thousand MGOLD.
Of course the value is ultimately up to the players, if we get oodles of "griefers" who trash the spot market prices by selling ridiculously cheaply feuds and such could end up arising as players interested in building up the value of the currencies try to suppress the ability of "griefers" to accumulate currency to "dump"...
-MarkM-