I also bring magnets with hooks, some real strong as I hung Christmas Decrations/lights on my door. And magnets with clips.
If I have a balcony I have a small thermometer with a suction cup that attaches to the outside of the sliding door, AcuRite 00799HDSBA1 00799 Digital Outdoor Window Thermometer, White. One caution is to remove it if your doing a B2B cruise as they my be hosting the outside area & it could come off.
Tom😀