Eat foods that are starchy, fatty and have a high sugar content. In other words, anything high in calories.
Here's a good combination: Make a peanut butter-banana-raisin sandwich. First you apply a generous amount of peanut butter. Then take a small handful of raisins and press them into the peanut butter. After that you take a ripe banana and slice it lengthwise into three pieces and place the slices on top of the peanut butter. It can be an open-face sandwich or you can put a top on it. This works well with whole grain pita bread.
With this combination you've got plenty of good fat from the peanut butter. You have whole grain bread and natural sugar from fruit. It's a safe bet to put lots of weight on the average person. And it makes a good evening snack.