Koblenz

Can certainly recommend the Vosges area. We were still campers at time and stayed in Oberbronn (on what was then a Flower Campings site). Nice area with lots to do.

Troyes: imagine venting your spleen at a fellow driver in your best vernacular Redingensian. It's in Burgundy but we've stayed near there. Pretty and half-timbered, to judge from the brown signs on the motorway.
Vosges: the s imperceptibly lengthens the o (might have been 'Vôges' elsewhere, perhaps) but otherwise disappears. My host family in Metz took me there for a big family party when I was 13. We ate at a huge table in the garden, looking out over the wooded hills. That's all I remember.

Someone mention mountain bikes? Slaver, drool...details required !

Good bikes. They'll enjoy them.

The Vosges is OK, though if you want a mix of things to do, the area between the hills and Colmar, (Actually in the Haut-Rhin, rather than Vosges department) is not a bad choice.

Towns like Kayserberg, Riquewihr, Turckheim and so on are attractive places to visit, and the cycling (though it can be a but uppy/downy) is good between the towns/villages via the vineyards (quiet enough to take kids on safely).

If you're into whites, the wines are spectacular (though not particularly cheap, even locally).

Colmar itself is supposed to be attractive (though I've never been) and Strasbourg and (for a little different adventure) Freiburg are OK for day visits.

As for the Rhine in Flames, if you're looking for a single overnight, then somewhere like Boppard, Sankt Goar, Bacharach, etc. would give you a taste of the Rhine Valley itself, and easy public transport to Koblenz (look for "party" or small-group tickets on the local transport, as, if they are available they save a lot of money).

The Rhine is just a little bit too noisy for me (trains and roads both sides), and if you were staying a little longer, I'd recommend going up the Mosel Valley, at least as far as Cochem, as it is less noisy (though still busy with tourists) and the local whites are very affordable (and good quality, especially Riesling). Cyling in the Mosel Valley is superb, and, as it is usually next to the river, very flat.

We find Germany considerably cheaper (all-in) than France, with eating and drinking out being a relative bargain.

If you are considering Germany, best think about pre-purchasing an emissions sticker (for which I can give you an appropriate, and cheap, online resource). 

 
 

Hey, another thing: Turckheim is where they make Staub. (That's 'Stobe', 'Vić, not 'Shtowb'.). Sven will know what I mean: enamelled cast iron pots and pans that make Le Creuset look like Fisher Price. I have, erm, four of their things already but there's always a use for one more - isn't there?

(I do make a bit of a habit of this. Last year we made a day trip to Thiers, where the beautiful elephant-badged Thiers-Issard knives are made. In 2013 we came home via the Normandy home of Mauviel copperware. And it's only just occurred to me that Cologne puts me a half-hour train ride away from Solingen, where the Güde company makes a bread knife with a blade 32cm long. Transporting it home through Eurostar security might be a challenge but one way or another, I'm going to have one.)

German public transport tends to be both efficient and good value if you know what you are doing.

The integrated transport system round Koblenz is described here:

www.vrminfo.de/

The zoning system, and the "group" ticketing arrangements make it cost-effective to stay out of Koblenz itself. A "minigruppenkarte" day-ticket covers up to 5 people on all public transport (train and bus) for the full day (though after 09:00 on weekdays) within the VRM area, and the cost appears to top-out at €22.10 after 5 zones. (that is €22.10 for the 5-person ticket, not each!)

This means that, subject to timetables, you could stay well out of Koblenz, up or down the Rhein, or indeed, along the Mosel with minimal travel cost. (most of the locations I mentioned are in the area covered).

Not a bad way of exploring some of the Rhein or Mosel, either.

(It's also worth noting that some local accommodation will also issue a free travel pass for the same area for the length of your stay - depends on whether they are participating or not, but worth a check! We've taken advantage of similar offers in Germany, France and italy).