If you have unused domain names, you can use parking companies such as Bodis or ParkingCrew to make some money out of it. You would be using their nameservers and it will display some ads on your domain names.
But you would be making penny unless you have some traffic to your domain names. I have more than 100 unused domain names but they don’t make me more than $1 per month. It used to be good few years back when PPC was high, nowadays they are not good. Parking is almost dead to be honest.