Steve - if you want to look at any of the WWI sites then if you have a sat-nav that allows you to import POI's then I can send you a file containing locations of pretty much every significant WWI memorial, cemetary, monument and museum in Belgium and northern France. I would counsel however, that you could easily spend all two weeks looking around those sites and never get down south! As Charles has said, Vimy Ridge is well worth a detour (take the guided tour of the underground tunnels, too), and if you do decide to take a look around the Ypres Salient then Tyne Cot cemetary (the largest CWGC memorial anywhere) and the German cemetary at Langemarck should be at the top of your list of places to see imo.
However, if you want to see the south then you are best off ignoring most of northern France and getting on with the journey. I would suggest:
- Autoroute to Reims
- Autoroute Reims -> Troyes
- N71 / D971 Troyes -> Dijon (a beautiful flowing road)
- Autoroute Dijon -> Grenoble
- N75 / D1075 Dijon -> Sisteron (the citadel is worth a look)
- N85 (Route Napoleon) Sisteron -> Grasse
My other tip is to buy the relevant Michelin maps and follow roads with a green edge as these are the scenic routes.
Unless you are in a hurry, think in terms of 3 days to get to the south coast, and longer if you want to explore. While you're down there a drive along both sides of the Canyon du Verdon is worthwhile, as is the coast road heading west, but avoid St-Tropez / Port Grimaud unless you have a masochistic streak and/or like sitting in traffic jams.
Personally, I wouldn't bother trying to get to Barcelona unless you have a specific need as you will miss out on some fantastic places in France. On the way back I'd suggest stopping off in the Ardeche (look around Aubenas, Privas, Le Puy) as there are some fantastic and quite quiet roads around there, then head north-west (Figeac / Cahors / Brive) and continue your journey homewards, keeping west of Paris. Just north-west of Limoges, stop off at Oradour-sur-Glane and visit the deserted village that has been preserved as a monument to the villagers who were massacred in a truly pointless Nazi atrocity towards the end of WWII. A very sobering experience.
Have a great time