If you feel like driving around in a rental car for a few more days, you can spontaneously extend the rental period at the local rental agency, depending on availability.
To do this, contact the car rental company directly. A follow-up booking with a new rental car from the operator is also possible, subject to availability. Please note, however, that you will then have to go back to the location to change your rental car.
Unfortunately, if you want to return the car earlier than planned during your rental, you will not normally be reimbursed.
Exception: If the early return is due to illness or other unforeseen circumstances, many providers will refund the costs on a goodwill basis. Unfortunately, you do not have a legal claim to this.