Gambling is actually only fun and exciting when it comes to real money. Only then do you feel the adrenaline and tension in your body. With the end that you hopefully win. You can also play with lower amounts, which you can easily lose, but I personally think that the tension is not there at all and then you can not really enjoy a certain amount of fun. It is difficult to say from what amount you can really enjoy it.
You might lose the purpose of fun if you lose big amount, and got exciting only if you get big wins or at least your balance increases.
There are people who play games without money, but that is not gambling of course.
This is often called demo mode and can be used to test a game, the results are different if using real money.