One thing to consider is creating a list of ideas -- picnic with the teacher, movies with the teacher, lunch with the teacher, etc. Then, you can send the list to the staff with a note saying the group would like to provide these experiences at the auction. Would anyone like to volunteer.
This way, you are giving them the ideas and they only need to volunteer for one.
Does anyone have a letter they give their teachers asking for "experience" donations for their auction event? Would love to see how other schools ask their teachers.