The weather can be quite changeable, so do go for a few thin layers and some kind of windproof top. Take walking poles, even if you've never used them before, unless you are a true mountain goat. On the slippery descents, if nothing else, they can really save you coming a cropper. This trip would actually be a good place to try them out for the first time. Also do make sure you have a decent level of fitness, it's a pretty tough week with some long days of walking. Also many people had blisters at some point so come well prepared - well-worn in decent boots, plenty of tape, blister plasters and make sure to protect your feet sooner rather than later. Applying Vaseline to your feet every day before putting your socks on really helps too.
On the free day you can borrow mountain bikes from the hotel - definitely recommended if you have any energy left (though not for the faint-hearted as those hills are pretty steep both up and down!). They have some helmets but given the state of the trails you'd be well advised to bring a good pair of cycling shorts and gloves if you plan to go mountain biking. Also, while the trip notes say you can go sea kayaking, this may not be available. Jonathan and Violetta, who run the last hotel, are absolute stars and very friendly - it's more like staying as guests of the family. They gave me free rein in the kitchen on the rest day to make a pack lunch out of anything they had, since I was going mountain biking.
Unless you go to the coast on the free day, you'll spend almost nothing on this trip, as there are very few shops, and you get some wine with the meals. I think I spent around £50 in total for the whole week, and that was basically on the two meals that weren't covered, and the guide's tip.
If you're reasonably fit and not afraid of steep hills or of being in fairly isolated surroundings, don't even think twice about this trip!