Do I have to limit to just one?
1) A SPORK ...I grew very weary of the limited breakfast options and as delicious as the spanish omelette is, sometimes I just wanted a yogurt. A spork would have come in handy sooner. It also worked well when I had a jar of peanut butter....which is a great thing to share with others, btw! Everyone said, yes, to having some peanut butter.
2) Hotel slippers. They are super duper light weight and feel good on the feet once out of the shower and walking around the albergue.
3) Slipper socks. Again, I like my feet being comfortable.
4) I was advised not to bring anything like a nightdress which in retrospect was advice I wished I did not take. I saw lots of women in lightweight summer dresses they put on after the shower. I plan to take a feather weight beach coverup which could double as a summer dress for when I get out of the shower. It's very modest.
5) Ibuprofen. OMG. A fellow peregrino who also happens to be a nurse gave this to me on my 2nd day. I literally was in so much pain I thought I had to end my journey. 4 hours later, I was doing cartwheels. She and this magic pill were my angels! The next day, I went to the pharmacy and bought some. I ended up sharing many pills with people who were also hurting that I developed a bit of a reputation as a "drug pusher" according to a guy in my Camino family. HAHA! Everyone was so thankful to have some and each then and went and bought their own supply. Crazy how most of us just didn't think to buy it to begin with....
As an aside, although I brought this, I think some may find it useful: I used 3 various sizes of the Sea to Summit waterproof bags, but I also brought along one of those super lightweight funky nylon totes they sell for about $8-$10 at department stores/boutiques. I often used it for my dirty laundry and kept my clean laundry in the larger S2S bag. It doubles as a purse and/or grocery bag... It weighs next to nothing.