You can probably just leave them with your hotel desk, they probably get mail and have out going just ask.
Agree to leave at hotel desk. You will also see mail boxes out on the street as you do your sight seeing. You can use any one of them to mail your post cards. In addition you can use Google to find an actual Post Office building near where you are staying. Just put your lodging address into Google and use the "search nearby " function to see Post Office.
You can mail post cards there too.
Where are you staying?
Use the USPS location finder and search for collection box.
Sometimes I just need a blue box too and they are getting harder to find.
But agree leaving at your hotel front desk is easiest unless you are staying at an airbnb.
Leave with the hotel front desk.
Every city usually has a Post Office. Look up the ones nearest to you. Most have a blue box outside near the street or parking lot. Or you can walk inside and drop off in mail slot. Did you need to buy stamps? We mix our letters and postcards in same box.
Agree to leave with hotel front desk. Avoid free standing mailboxes, as the entire mail box can be stolen which happens more than people know.Edited: 6:48 pm, November 25, 2017