One of my favorite tools is the homemade mass gainer. They're typically a lot more nutritious and cheaper when compared to the powdered form in the huge bag. A lot of the size I've packed on over the years is directly attributed to homemade mass gain shakes.
You need a blender, doesn't have to be anything crazy. I have one of those little Nutribullets but have also used one of those powerhouse Ninja blenders.
This is a great way to get a shitload of calories super fast, which makes it ideal for a post workout meal or just a meal replacement in general.
Ingredients are extremely variable and you can make these as lean or as dense as you'd like, here's what mine typically consists of:
1 scoop whey protein
1 cup milk
2 tbsp peanut butter
1/2 cup oats
1 banana
Little bit of honey
Little bit of ice
Alternative ingredients: spinach/greens, greek yogurt, cottage cheese, frozen fruit, almond milk etc. The sky is the limit! You can easily down 1000 calories or more in just a few minutes, or sip on it over the course of a couple hours. Let me know what you all think.