Trader Joe's is your best friend for this!! I buy "Just a Handful" nut varieties...mainly Roasted Almonds. I can easily store them in my drawer and eat some when I'm hungry. Good source of fat and protein and helps to give that little boost of energy when work gets sluggish.
I also bring a bag of veggies daily to snack on (broccoli, grape tomatoes, chopped celery). Yes, everyone knows me as "The Healthy Guy", but I would never take that as a bad thing :)
Just nuts is hard because of the tendency to eat too many. I keep a tub of Kerrygold butter and use my almonds to dip up a good dollop of that for a quick satisfying snack with just enough fat to make me happy with only a half dozen or so nuts.
I keep Macadamia nuts in the freezer, they have the best texture when they are frozen--cold and chewy. They taste great frozen, stay fresher for a longer period of time and the office food thief never thinks to look in the freezer ;) I also throw in a few Brazil nuts for variety.
make bacon at home - bring it into work in little baggies. No need to reheat.
hardboiled egg with sea salt and cracked pepper (yes. bring in a pepper grinder. nobody cares)
small baked potato with butter and avocado slices
almonds or pecans or macadamia nuts or a mix of all of them
dried figs (I like black mission figs)
sliced aged cheese (Kerrygold Dubliner is a good one.)
a handful of blueberries or a banana sliced up in a bowl with heavy cream poured over top
Keep in mind that many of these items can be combined very nicely. So for example, some sliced cheese with dried figs and some pecans. or maybe some bacon wrapped figs. or maybe some bacon smashed up in a baked potato. or you can toss some pecans in with your strawberries and cream.
I pack my husband a "lunch" everyday for work. I'm more of a 2-3 meals a day girl and he's more of a snacker so I pack him a lunch and snacks for the day. The highest fat cottage cheese is a good option if you do dairy - It's low in carb, high in fat/protein. I give him half of a granny smith apple and 2 tbls. of sunbutter. I chop up celery and carrot sticks. Katamata olives are good, or probably any olive. Hardboiled eggs, a slice of cheese rolled up inside of a piece of good quality lunch meat. He craves salty/sour stuff so I sometimes give him a pickle and some pickled pepperocini peppers. I know these aren't all "paleo approved" but you gotta find what works for you.
I make up about 1-1.5 lbs. of homemade turkey sausage (1.5-oz patties) on the weekends and put them in the freezer at home. I'll bring some to work and when I hit that midafternoon slump I grab one of those from my office mini-fridge and eat it along with a handful of sugar snap peas or some coconut milk. It's usually enough to tide me over till dinner. Sometimes it'll be an one-ounce or so serving of meat from the previous night's dinner instead.
The bonus is that instead of trying to finish off every last morsel I cook for dinner, I'm stopping when I'm full, knowing that this little bit of meat isn't enough for a meal but is perfect for a snack.
It helps to have your own mini-fridge at work, but the office fridge will work too if there's no food thief.
I keep a large bag of loose green tea at work... on fast days I probably drink more green tea than I should. If I'm forced to snack, I'll pack a lunch and peck at it all day... usually it's steak (or some other leftover) sliced up into bite size pieces with lime juice, maybe some hard cheese (manchego or irish cheddar), some macadamia nuts (raw and unsalted), and berries.
I drink coffee (decaf for me) with some organic heavy cream and cinnamon.
Homemade fat bars (coconut oil, coconut butter, dried sugar free cranberries, nuts all mixed and refrigerated in a pan, break into bark)
Small Paleo Kit
Of course it helps we have a fridge or two in the office.