Decorate your door facing the outside hallway with items that you don't care much about. Because the items are facing the hallway, they are at risk to be damaged or even stolen.
Create a blank canvas for people to decorate. Cover the door facing the outside hallway with a colorful piece of butcher paper. Attach it with tape or adhesive that will hold it in place but come off easily. Buy some inexpensive markers. Attach pieces of hook and loop tape to a few markers and attach those markers to the butcher paper. This will allow people to write messages and draw pictures with the markers.
Create a yearlong collage on the inside of the door. Attach a large piece of butcher paper or several pieces of poster board to the door. Collect items throughout the year and tape them to the dorm door. Tape to the door photos, postcards, souvenirs like ticket stubs, notes from friends and other items that you may obtain throughout the year.
Tape pieces of twine or string across the door inside your room. Hang them in a clothesline fashion. Attach clothespins to the twine and attach photos and other items to the clothespins. Change them as often as you like.
Look for removable vinyl stickers in a pattern you like at a craft store and attach them to your door.
Use posters, maps or printed art to add visual interest to the door.