Doctor's Orders: Rest Means Nutrition
Often, bodybuilders think of rest days as time away from bodybuilding. Nothing could be further from the truth! You don't grow when you train; you grow from your training while you're recovering.
Your body can only effectively recover when it's not under assault by weights. A non-training day is also an ideal opportunity for you to load up on quality foods, but be vigilant with your diet. Don't take in more calories than you need.
Focus on lean protein and quality slow-digesting carbs, and give yourself a huge edge in building mass.
This goes for supplements, as well. Obviously you won't need your pre- and post-workout shakes on rest days. However, I suggest that you still get in one dose of creatine and beta-alanine, even on rest days, with your morning shake.
Remember that every rest day should be an active rest day. Rest days are for recovery; they're not an excuse to be lazy. Get out and hit 15-30 minutes of HIIT cardio today, or perform 30-60 minutes of your favorite activity: hiking, biking, walking, playing a sport, etc.