Get some protein powder and have a shake, or add it to oatmeal or something. It's a good way of adding calories without having to eat more. Otherwise, milk can be high in calories and good for you as well.
Drink juices to help get your calorie intake up. Just make sure it is juice and not just Kool-aid type drinks. Lots of "juices" sold in stores are actually largely sugar and additives. Juices can be good because they give you your nutrition requirements- and in your case can elevate your calorie intake
You also can eat almonds etc. when you are not really hungry but it would be time for a few kcal best fats, lots of fiber, vitamins and a hand full of this is about 250kcal.