If you neither gain nor lose weight, you should be able to add a couple of weight gainer shakes and pick up some lbs. I used to mix my weight gainer shakes with PB, ice cream, whole milk, salmon--wait, maybe not salmon, but you get the point. Now before I get destroyed for saying any of this, I would like for everyone to know that this way is NOT OPTIMAL. Whole foods are best, but for anyone who is busy as hell, and barely finding the time to breathe, shakes help.
Any weight gain shake will do. This is my recipe that had me on the treadmill within 3 months: However many scoops of that sugar garbage you bought, whole milk, scoop of ice cream and 1 scoop of whey protein, tablespoon of PB. Ideally, the calories/grams break down like so: carbs-600/150, fat-360/40, protein-300/75. Grand total of 1260 cal. Best if you split it in two. Best if consumed right before bed.