3. Make Your Own Smoothies
Smoothies are a great way to mix together all kinds of good stuff to gain weight healthily that you would probably rather not eat individually. The best way to make sure your smoothies are full of good stuff, and no unnecessary bad stuff, is to simply make them yourself at home with ingredients you feel comfortable consuming.
You can add a bunch of the other foods suggested here in this list, but what you want to focus on mostly is making sure your smoothies are high in protein. More specifically, try adding in nuts and seeds, natural nut butter such as peanut, and dried fruits.
One of the main benefits of dried fruits as opposed to fresh or even frozen versions is that they have dramatically less water in them. Adding water from fresh or frozen fruit to your smoothies does you no good if your goal is to gain weight. Also try whole grains such as oats, as well as full-fat milk and/or yogurt.