You sound like you would be responsible enough to have a cash back credit card that you pay off in full every month.

Try to build up your credit to where you qualify for something like the citi mastercard 2% cash back. Hoard those reward points until cybermonday and buy gift cards at a discount on something you would buy anyways which could be a hit or a miss although you can always use those reward points to cash out or offset your bill.

As for not stressing about spending money just focus of the value buys that cost more but last longer. Besides that start saving for retirement by opening a roth ira and seeing if your employer offers a 401k match.