Love CHECK24 and use it for all my rental car bookings. You can book way in advance and take advantage of great special price offers. If plans change you can easily cancel up to 24 hours before pick up. Only downfall is that you can’t easily change pick up or drop off time once you’ve booked. Easier than changing time is often fully cancel the booked car and book new one, which is unfortunate. So make sure you know exactly when you want to pick up your rental.