Swiss Army knife. Anything that will get you into any known food or alcohol container is worth it's weight in gold, let alone a tenner.
OS maps are truly a thing of beauty. Really, they are.
The humble bungee. Lost without them.
The Logis de France website.
And finally but most importantly, long downhill roads with a tailwind, through stunning scenery with good company. The road down the west bank of the Rhone is a good one. Not too busy, enough villages to keep you fed and watered, wined and dined, and goes past the Cote Roti, Crozes-Hermitage, Chateauneuf du Papes and down towards Mont Ventoux. Utter bliss.

*goes to pack. If only...*