If they don't already have one, a nice personal sign that they can hang off the hitch of their 5th wheel or stick in the ground when camping at various parks. As we've traveled around we've seen plenty of the wooden signs with someone's first names and possibly their home town & state along with something decorative on it.
Don't use their last names, especially with their hometown indicated. This could alert criminal types that there's nobody home and set up a potential break in.
Nice gravity lounge chairs are a great addition to a 5th wheel as there is usually sufficient storage area for them. They are very comfy for lounging around the camper after a day of hiking or sightseeing.