Parkn'Sleep is made for spontaneous travel: you drive there, check in on site and pay digitally. The app shows you the live availability of every parking spot, for example 7 of 10 spots free.
More and more hosts offer premium parking spots alongside their standard parking spots. You can book these for today or a later date. Your parking spot stays reserved for you for the whole stay, even if you drive off in between.
You'll find everything else under Booking a parking spot in advance.