If you like wine and fireworks best time to go to direction Koblenz is in September. There's a wine-festival where many local wines can be tasted. It takes place in St. Goarshausen, which is situated south to Koblenz, right next to the Rhine. The wine-festivities last one week and at the last day (Saturday, I think) there is this gigantic firework known as "Rhein in Flammen" (Rhine in flames). It's absolutely amazing and best: it's free. However, you should be early to find a nice place from where to watch the firework at night. If you've got too much money ;) you can buy a ticket to watch it from a ship.
(Note: There's also a Rhein in Flammen next to Bonn)
You will love it!
This year: 17. - 20. September (NExT WEEKEND!)