Chocolate. There is an infinite variety of flavors, and many come with nuts, fruit, coconut, etc, mixed in. Also, it is high in calories, and many varieties contain substantial fiber.
Organic Vegan Food Bars and Raw Revolution Organic Food Bars
are both tasty and healthy-lots of fruit, nuts, etc, ground up and formed into soft, chewy bars. Along the line of a Pro Bar, but in different flavors.
Crushed potato, corn, or multigrain chips. Lots of varieties, all fairly high in calories. If you crush them, you can get a lot in a small space.
Energy drinks such as Perpetuem to sip on the move and drip the calories into your system without bothering to sit down and fuss with wrappers, etc. I can go all day on the stuff.