The Explorer it GREAT !!. I have been on the Explorer and Adventure. The formal nights are very dressy to formal (Dark suit or Tux). You will find some people with regular suits also. Alot of people just dress for dinner and then go back and change into casual cloths after dinner. Last year we had 2 formals, 1 dress casual, and 4 casual nights for each 7 day cruise. The ship does have valet service for a fee so if you need somthing cleaned they can do that for you so you can get away with re-wearing one weeks outfits the next week since you probably will not be with the same people anyway. The ship is outstanding and provides for a wide variety of activities for people of all ages and interests.
Have a great time !!