Welcome to our hobby. While you didn't say what your new "classic car" was, we'll try to share what we do. Our "classics" are primarily Pre WWII cars and we travel often with them either driving or trailering them. Leaving them unattended is always a concern. We avoid restaurants that are primarily drinking establishments, we avoid dark parking lots, we try to sit where we can watch the parking lot. End spots are safer because your car generally can only be bothered from one side.
Expensive radiator caps can be removed and replaced with the low buck plane one when parked.
At motels or hotels I seek a well lighted space visible from my room, cover the car, and leave my sign on the cover, "If you can read this, you are in range". Sleep comes hard. Some times if you ask, you can leave it in the porticoche..
Good Luck, and send us a picture of your "classic".