Ryanair currently offers cheap return tickets on its direct route from Dusseldorf to Zadar. The airlines if flying to Zadar from Weeze Airport, but due to the low price this may be interesting for some passengers which wants to visit mentioned cities. Cheap flights are now on sale for both ways, departing from Germany or from Croatia!
As we mentioned, cheap tickets can be purchased in both ways for few dates in September this year, here are links for specific dates and departing point:
Dusseldorf - Zadar - Dusseldorf (September 2020.)
Zadar - Dusseldorf - Zadar (September 2020.)
If you want to travel on this route, be sure to book the ticket ASAP since the price is really low and may be sold out soon!