With no hesitation, I'm going to jump right into my first (or technically second), right away!
Starting with probably one of my favourite diets to assist gaining weight, the GOMAD diet!
GOMAD, or Gallon Of Milk A Day, is a popular diet for those seeking to gain weight easy.
Now it pretty much lies in the name, gallon of milk a day, or around 3.7 liters (I usually round it off to 4 liters a day) if you want it in the metric measurements.
How you do it: Go to the store, buy a gallon of the milk of your choice (pref. Whole Milk, but ~1.5% is fine too), and consume it in during the day. If you are getting the whole milk you are getting a whopping 2400 kcals per gallon! That is most people's caloric needs for one day! Now it does not end there, you are not only going to consume milk, but you are also going to eat your regular meals, so breakfast, lunch, dinner etc.
So that means almost 5000 kcals for one day, with that you are guaranteed to gain weight. Though, there is a downside with consuming that many calories in one day, you are most likely going to gain fat, so what you could do is cut down maybe to half a gallon (~1200 kcals), or you could get 1.5%, another thing you could do is to cut down your regular meals.
Either way, to minimize fat gains you should aim at getting in 3500-4000 kcals per day.
All in all GOMAD is a simple and cheap, but yet effective way to gain weight and also get important minerals and vitamins that your body needs.
NOTE: I do not reccomend this diet for induviduals who do not take part in any phycial activity (lifting weights preffered), as the outcome may not be the best thing for your body composition.