The tourist advertisement "Just know how to play" is translated into English. There are two alternatives, Just for fun and Just fun, which is more reliable?

just for fun means to say it from the consumer's point of view for entertainment. That is to say, this place is very interesting and I want to attract more people to have fun here.

just fun, generally speaking, is not said this way. If for is not added, it is suggested to say Just Fun, saying that this place is interesting, from the perspective of a bystander.