I want stuff to snack on that’s tasty but takes a lot of work to eat, so that I can’t eat very much of it during a snack break. Sunflower seeds (unshelled) seem like the most obvious example – it takes me about 5 minutes to eat 20 sunflower seeds. Bonus points for shelf-stability and wide availability. But I’m worried I’m going to get sick of sunflower seeds.
What are some others?
If it takes you that long to eat sunflower seeds, you can probably manage 2 Pistachios.
Edamame. They are soybeans in the shell. Very yummy, lower calorie and self-limiting due to effort.
I like edamame, but I can eat those suckers fast – just pinch with my teeth and pull.
There’s a joke about celery that you get fewer calories from eating it than you expend in chewing it. It’s not really true, although the stuff really is low in calories.
Have any hickory trees around? Great nut, pain in the ass to get the meat out of.
Yeah, but celery sucks. 
Brazil nuts. With no cracker. That’ll keep you busy.
Black walnuts are even harder to crack than hickory nuts. There are special nutcrackers made just for them.
Well, it is only soybeans and some sodium, protein basically. Splurging won’t set you back much.
Most nuts unshelled. Almonds, pistachios, peanuts.
I eat a few almonds every day. There ara lot of health benefits associated with them.
Oranges take awhile to peel and are low in calories.
Eat half the slices as a mid morning snack
Eat the other half as a evening snack.
They stay good in a zip lock bag for at least 12 to 24 hours.
Potato chips. Just run 100-200 yards between each chip. 
Sometimes I get up and walk into the kitchen and eat one peanut, or one chocolate lchip, or one raisin. Then I go back to what I was doing. Thats a pretty good ratio. Sometimes I just find one under the sofa cushion, which is not so good.
Whole coconuts.
Dennis
Artichokes? They have several advantages: they are low in calories and fat, they take a long time to cook, take a long time to eat and are delicious. Disadvantages: they are expensive, and they are not a traditional snack food. 
6 mayo-based
2 butter
1 peanut butter
1 Greek yogurt
Only the Greek yogurt is potentially lo-cal.
I actually like it, but try putting some peanut butter on it. The good stuff, not Skippy. Or try almond butter or humus or something.
Artichokes are delicious plain if you boil them in salted water. But probably not what the OP is looking for!
This would be a great idea, except that I don’t think I could open them without disturbing my office mates. Plus they’d make a big mess!
The peanut butter would kind of ruin the low-cal goal – maybe hummus would work, but it’s not shelf stable, I don’t think (i.e. I couldn’t keep it in my office desk drawer).
Yeah, this is really an office-snack, and meant to fit in my bottom drawer at work. But thanks!
(Referring to coconuts) I saw, one time, a Warner Brothers cartoon about a squirrel living in a city park. He scampers across the street and finds a nut vendor with a sidewalk display. He grabs an armload of peanuts then, before he can make it back to the park, spots the almonds. Disdaining the peanuts he grabs the almonds, spots the walnuts and switches to those, then sees the coconuts.*
The rest of the cartoon is him trying to open the one coconut he filched. Gnawing doesn’t work, getting a truck to run over it doesn’t work, dropping from a great height off a building doesn’t work. Even applying a jackhammer makes the tool disintegrate instead.
Finally, exhausted as the sun is setting, he dejectedly puts the coconut on top of the pyramid he stole it from. It promptly rolls off the pile, hits the sidewalk, and splits. Eagerly, he wrenches the shell apart only to find another shell underneath. One of the eyes in the inner shell winks at the fourth wall as they iris out.
*I know, I know; but you buy the joke, you buy the premise.