A side table. Guests do more than sleep, especially if they're staying for a few days. A little table for applying makeup and writing letters (OK, checking email on the laptop) is a useful addition to a guest room.
Wallpaper is a great way to bring the feeling of nature into your home without actually putting plants in the house. If you're allergic to plants (or just have a brown thumb) then a nature-themed wallpaper choice can help you bring the outdoors in.