We are looking for the best flights between these cities. Any ideas are appreciated
Check whichbudget.com and flylc.com for discount airlines.
Lee, Wizzair offer that service from Frankfurt-Hahn, which is closer to Bernkastel-Kues (Mosel river) than to Frankfurt. There are €12 one-way shuttle bus services to Hahn airport that take about 2 hours. Maybe you can incorporate the flight to Krakow to your Rhine/Mosel river itinerary?