Booking Flight:Destination Address

I would give the hotel you have reserved for your first night in the country, together with your mobile phone. Your luggage may be misdirected enroute, and the airline needs to know where to send it.

As a retired US Immigration Officer, I can categorically advise you to simply use the address of the first lodging you plan to stay in. Even if you are planning to stay in an albergue, use that street address. You can always write "c/0" (care of) in the address.

The purpose is so that the authorities would, ostensibly, know where to find you should they desire to. Even if you are gone, like on Camino, the authorities would have a place to start to locate you. The US has been collecting this information for decades. European countries are slowly starting to catch on. Some use the airline reservation systems to collect the information as early as possible.

Just as an aside, there is a valid reason to collect this information. I was personally involved in using this information to locate a suspected terrorist, post 9-11. A person of interest had entered the US heading towards a flight training academy and was discovered at the second hotel they registered at. The address provided for the first night's hotel was correct, but the person had checked out by the time the FBI arrived. Fortunately the person made a second night reservation at another property of the same chain in another state. Doh!? That story had a happy ending...

I hope this helps.