How should I spend my money wisely?

Currently living in Tokyo but I’m working a long distance software engineering job. I make about 6,000€ a month give or take and I spend only 2,000€ on rent, utilities, food, going out and etc so I have 4k every month just sitting there. Don’t think having it in a bank account is smart. I’ve hear about index funds or whatever and my old man also said I should try buying gold or something. Considering I’m in my 20s it’s alot of money I’m tempted to spend. Thoughts?