Here is a basic check list for both day and multi-day (self-guided) walks:
Small backpack Detachable trousers
Water bottle Sunglasses
Sun hat Sun cream
Small first aid kit Compass / GPS
Torch Waterproof (preferably Goretex) jacket and trousers
Walking Boots (broken in) Mobile phone (and emergency numbers – provided by Basque Country Walks)
Walking stick Gloves
Penknife Insect repellent
Food Fleece
Binoculars Whistle