Do not pack for a just-in-case scenario or you will weigh yourself down. With a pilgrimage in mind, it is easier to travel without conveniences that bring weight and distraction into the equation. On a pilgrimage less really is more.
One reason to pack only essentials and keep your backpack light is to protect your feet. Foot injuries range from simple strains, bruises or blisters to more complex problems such as breaks, sprains or torn tendons. Even blisters, a common complaint for pilgrims, can become serious if not treated properly. As you walk along the Camino you will become increasingly aware of your feet. Hiking boots provide stability and protect your ankles while you walk. Choose a second option like sandals which are lightweight and help your feet relax and breathe after a long day. They can also provide a dressier option if needed. Other items for your feet include extra socks, a blister-treatment kit and anti-fungal cream.
Pack to protect your head and body from the sun. A bandanna is a lightweight and versatile item, so take at least two. Pack a small container of sunscreen and take sunglasses and lip protection. Bring empty bottles for drinking water.
Pack the kind of clothing that makes sense given summertime and climate change. Clothing made of light breathable fabric (not jeans) and a few changes of clothes will serve you well. Linen or cotton pants, shorts and tops wear well. Some women bring a cotton skirt, which is versatile. Even in summer a light shell or jacket may be helpful for chilly mornings.
When you pack sleeping gear be sure to include the following: lightweight sleeping bag, travel pillow, ear plugs, nightshirt, mosquito repellent and flashlight or headlamp. These are basics that will enhance the quality of your sleep.
Place your toiletries in zip-lock bags, bringing enough for a week. Plan on replenishing your supplies as needed -- all your essentials can be bought along the Camino.