Congratulations - nice trip you plan!
My comment and suggestion: Plan some more time and drive from Cologne along the river Rhine to Wiesbaden and on to Frankfurt. The stretch between Cologne and Rüdesheim am Rhein is full with medieval castles - several dozens it is already a UNESCO World Heritage. Would be a real pity if you missed that.
Even from Japan tourists come in droves to see this part.
As it is a "Bundesstraße" (major road) be prepared that lots of tourists and excursionists are on the way - particularly during the holiday peak in summer and generally on week-ends. You may lose time. On the other hand there are scenic view points over and over again. At least at some you might like to stop and enjoy the romantic-medieval scenery.
Being there you should take the time to visit one of these castles; if possibly with a guided tour. A very good and genuine insight into the medieval life of the knights on the castles is given at castle "Marksburg", over the town of Braubach. (A few kilometers east of Koblenz). It is the only castle that has never been destroyed and which looks (almost) 100% as it looked like some 500 years ago! You will see the weapons, armures from different periods, the buildings from inside along with a kitchen, dining room, living room etc., the panoramic view and - yes - the torture chamber. (Sweet dreams next night!)
And make sure that at least from Koblenz on direction to Frankfurt you take the left side of the river Rhine, road B42. From there views are even nicer than from the other side (B9).
Some 70 kilometers after Koblenz you might face your first challenge: The railway crosses B42-road right at the western entrance of the (highly touristy) town of Rüdesheim. High traffic of tourists and the ferry boats that debark cars there may lead to time-consuming traffic congestions.
Right after Rüdesheim, heading east still along B42, comes the next point you should pay a visit to: The town of Geisenheim. It is nothing special (for tourists), but it is my dear home town.
Am always glad to find EBTers leaving and getting bills there... You would be the first EBT-visitor from the Iberian peninsula.
Your time to plan: Without any visit to any castles and mainly driving, calculate with at least around four hours to get from Cologne to Frankfurt. Well, depends on the season and the traffic if it will be less or more.
Forget about travelling along the river Rhine between Koblenz and Rüdesheim (both sides of the river) during the last sunday of June. Traditionally, both roads are blocked that day. Access is only allowed for skaters and bicycle drivers. I cycle this road almost every year because it is sooooooooooo beautiful!
Should you be around Rüdesheim at noon time better find yourself another town for lunch. (Geisenheim?) Prices are touristy in the restaurants and they might be full anyway. All the other towns east of Rüdesheim are quite void of tourists. Prices are lower and parking lots easier to find.
In this wine area between Koblenz until Wiesbaden you might feel seduced to sip a small glass of wine at the event of an informal wine testing. Apart from that no-go of drinking and driving, please be aware that our wines are a bit strong! Some friends of mine from abroad or from other German regions have already learnt quite a lesson (with quite a headache....)
Okay, that's it first. Depending on which day you will pass our region and if you might change your plans completely and stay for a day, let me know. We might meet and I can show you some things. In any case you are prone to get a very interesting EBT-map.
Meine Angst, dass die Autokorrektur einmal etwas Obszönes ausspuckt, wichst täglich.
I've made a Google Map that anyone can edit (please don't change the lines, though), so feel free to add placemarks in places of interest, it would be great! 
)
)
