Doctor's Orders: Eat!
If you want to add size, you have to eat. You need to emphasize quality foods like chicken, lean red meat, fish, eggs, dairy products, brown rice, oatmeal, and sweet potatoes, but you can also use foods and supplements that are easier to get down. This is an excellent time to consume mass-gaining beverages such as protein shakes, with or without carbs. You can even add an occasional quart of milk to your diet. Liquids make it easier for you to take in the calories you need for adding body weight.
If you find it hard to consume enough calories throughout the day, consider drinking casein protein shakes between meals. Research shows that casein may make you feel less immediately full than whey protein and allow you to eat more whole food later on.
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.