Though I'm on the complete opposite end of needing extra calories, my suggestion would be to NOT indulge in the high, empty ones (you know, sugary stuff, like Nutella or cookies). Sure, it's got a lot of calories etc, but no nutrition.
Try things like nuts and nut butters (celery and PB tastes really good!), avocados, cheeses (maybe cheese sticks or those little Baby Bell circles?), dark chocolate, trail mix...