Where in the western US could I fly to inexpensively from Detroit?

My friends and I are looking into going on a trip out west either during Christmas break or after school ends in June. I am looking for flights under $300. What would be a good destination in the western US for us to go to that we could fly to inexpensively from Detroit, Michigan? We don't want to go to Las Vegas; are there any other places that offer cheap airfare from Detroit? Thanks!