Less is more. First, it’s perfectly fine to wear the same thing day after day after day. Secondly, be ready for rain with a waterproof parka, rain paints, and appropriate footwear. We preferred old-fashioned, high-top leather mountain boots, a bit heavy but lessening the chance of twisting an ankle, as well as good for slogging through mud. Speaking of mud, it has a habit of clogging up a boot’s cleats, and sending you slipping and sliding. For which there is a great solution: the support and balance offered by collapsible walking sticks (which as well take considerable pressure off your knees). Finally, remember that even in May or September, it can be cold, even snowy at higher elevations. Have a plan for layered warmth, and don’t forget gloves.
When staying in gites (France) or alburgues (Spain) – with their rudimentary bunkrooms (and the possibility of bugs) – a lightweight sleep sack (less than a pound) is useful, as is a small hand towel (though at first a T shirt sufficed). And a bit of under-the-cover squirming is in order when changing clothes for the night and in the morning.
Phone. We were happy with a tiny, basic eKit (provided by Eurail). Good service, with available minutes remotely added as needed. Though not essential, iEverythings can have a role in keeping a journal, checking email, and reading books.
Camera. Compact, lightweight, under $200 Lumixs, Sonys, or Nikons can’t be beat – and can provide material for a stunning photo book or PowerPoint back home. A modest 3X zoom will cover just about everything. Very helpful: a little, pocket tripod, good for self portraits and up to a second or more time exposure if braced sideways against a wall or pillar at nightfall or in a shadowy church.