Aukey’s simple magnetic smartphone vent holder is one of the most popular car mounts out there, and from personal experience, I can tell you it’s awesome for taking on vacation for use in a rental car as well. For $4, go ahead, give it a try.
If you don’t want to block a vent, or just want to keep your phone closer to eye level, Anker’s low profile magnetic dash mount is also on sale for $12.