Actually, my most profitable financial plan is having a credit card, getting most of the interest-free period and a credit card bonus while keeping my paycheck on my account with a small interest rate for as long as possible. It may sound funny, but I make like 100 crowns a month on that[1].
[1] Now, some may argue someone is paying for all that, which is true, but it's all of you poor people actually paying interest on the credit cards. Sorry and thank you.