Hez,

Sounds like a blast, but it would be expensive at best and potentially impossible at worst. Most European rental cars have a three day minimum. I guess it's okay if you plan on keeping the car for three days, but for a Rhine cruise, I'd use the train.

You can take a train from Frankfurt to Bingen (start point for most scenic Rhine crusies) for about 15 Euros a ticket.
Get there early and you can hope on/off at all the scenic towns along the route. You could spend the day cruising, visiting little towns, and return to Bingen on the same evening. If you looking for something longer, you can easily cruise to Cologne and spend a night. If I went to Cologne, I'd return to Frankfurt by train.

If you plan on renting a car for three days, I'd get one from Frankfurt Airport (as cheap as $23 bucks a day on hotwire.com) and return it at the airport. I'd hit places like Heidelberg, Speyer (personal fav), or head towards the Mosel and visit Berg Eltz, Cochem, and Trier.

Here is the USA link to Deutsche Bahn if you want to play with train fares. It's really easy to use:
http://www.bahn.com/i/view/USA/en/index.shtml

thanks agedude - that's some excellently useful advice.

So I checked on Avis and I can hire a car on Saturday in Cologne and return it Monday at the airport in Frankfurt for $156 (Mercedes B Class). I do have top status with Avis so that probably helps. Does that make sense to do? We were thinking to see a bit different scenery on the return route.

Given this as an idea, is there a great route to drive and/or a great place to stopover on the way back?

Thanks,
Hez

Does that mean you intend to do the cruise all the way to Cologne? There are not many cruises that do the whole way, you would spend something like 10 or 12 hours on the boat, and apart from two short stretches the landscape North of Koblenz is not of interest so this would be really long and tedious. Koblenz is the northernmost logical point to end your cruise.

Combining cruise and train is IMHO a more sensible approach. This turns out a lot cheaper, you don't have to reach a car rental somewhere in the suburbs, and all of you can enjoy the landscape on the way. Otherwise the driver has to concentrate on traffic (these are busy two-lane country highways) and won't be able to watch the scenery. Local trains run at regular intervals and stop in all towns and villages along the way, so you can do as many stopovers as you like.

"I can hire a car on Saturday in Cologne... Given this as an idea, is there a great route to drive and/or a great place to stopover on the way back?"

I'm a little unclear what days you have. Fri through Mon?

Indeed, Bingen is the best place to start your cruise. A ticket there for two costs 31€ from Frankfurt proper (Frankfurt a.M. station,) 25.70€ if you're starting from FRA ("Frankfurt a.M. / Flughafen / andere Haltestellen" station:

http://www.rmv.de/auskunft/bin/jp/query.exe/en?application=TARIFFONLY

(Select "season ticket ohne Umweg", then look for "Gruppentageskarte 5 Personen" to find the ticket price.

I would actually suggest ending your cruise prior to Koblenz; the scenery as you approach Koblenz becomes a bit industrial, and the dock in Koblenz is a LONG walk from the station. End it in Boppard instead (nice town to stay in with a good choice of hotels and restaurants) or a bit further north in Braubach (home of Marksburg Castle) and you'll have gotten a glimpse of the best part without much dead time on the boat in 2.5 - 3 hours.

Be sure to show your train ticket at the K-D dock for a discount on the cruise.

The Middle Rhine Valley offers a lot:

Castles: http://www.welterbe-mittelrheintal.de/index.php?id=288&L=3
Villages: http://www.welterbe-mittelrheintal.de/index.php?id=274&L=3
Nature walks and scenery: http://www.welterbe-mittelrheintal.de/index.php?id=230&L=3

Then if you're headed to Cologne that night, board a train.

However, staying in one of the smaller old-world villages, whether Braubach, Boppard or one of the others you passed cruising north, will give you a much better taste of this unique area than busy and mostly modern Cologne, and it would be easy to fill a weekend in the other Rhine and nearby Mosel valley towns. Since you don't already have a route to explore before you return to Frankfurt, do check out the Mosel:

http://www.mosel-reisefuehrer.de/moselhighengl/moselhighlightsengl.html

www.burg-eltz.de


I'd postpone the car decision until you know where you want to travel. The Rhine has trains running up and down it on both sides of the river all day long, and the Mosel is similarly accessible. You could do this area by car, but it would cost a good bit more, and the trains are great. Service is frequent, dependable and simple. You can get a 3-day pass for 2-5 people for 40€ that covers Rhine train travel from Oberwesel in the south to Remagen in the north, and Mosel travel from Koblenz west to Cochem (great town) and Bullay. The villages on both rivers, with the exception of Koblenz, are easily walkable. But the 40€ pass also covers buses within Koblenz and elsewhere if you needed them. See the 3-day mini-group ticket details here:

http://www.vrminfo.de/en/tickets-and-fares/ticket-offers/leisure-ticket/

Burg Eltz Castle mentioned above is tops and is accessible on Sat or Sun taking Bus 330 from Hatzenport (on the Mosel river) train station or on foot from Moselkern station:

http://www.vrminfo.de/uploads/tx_vdvtimetable/330_01.pdf
http://www.bensbauernhof.com/burgeltzfrommoselkern.html

Thanks so much for all the suggestions!!!

My husband is really not into trains as he is a mad keen driver so I think we're going to stick with the cruise one day and then driving the others. I was able to get an awesome deal on a Mercedes SLK Roadster from Sixt in Mainz which turned out to be fortuitous as Mainz seems like a better first/last night option than Frankfurt.

So - here's the plan now...

Wednesday - I arrive to Mainz in the evening from Heidleberg. Overnight at the Hotel Hammer in Mainz, right across from the train station.

Thursday - DH arrive at FRA at 5:30am from Johannesburg (business class so hopefully he will have had some sleep!). He will take the train to Mainz and meet me at the hotel. After shower/breakfast we will take the train to Bingen and cruise, probably to Braubach to visit Marksburg Castle. We'll then take the train back to Mainz.

Friday - pick up the car and drive to Koblenz via this route - http://goo.gl/maps/8v33 - which is based on an itinerary from Karen Brown - http://www.karenbrown.com/Trip_Planning_Itinerary/Germany/Castles_of_the_Rhine_and_Mosel_/36.php. Overnight at http://www.hotelfriends.de/alle-standorte/mittelrhein/

Saturday - again using the Karen Brown itinerary - http://goo.gl/maps/8n3l - hotel options were a bit limited with the Roman festival being on in Trier I guess so I've booked the Parkhotel Bad Bertrich (http://www.parkhotel-mosel.de/) which looks to be quite nice and only 10 minutes or so off the Mosel.

Sunday - probably continuing on to Trier and then back to Mainz. DH might have to fly from FRA to Prague for business on Sunday night so this day is a bit open at the moment. Ideas on a route from Bad Bertrich that ends up back in Mainz would be most appreciated. I, at least, will spend the last night at the same hotel in Mainz.

Thanks again for the help. Most appreciated.