Corny as this sounds:
Box of cigars. Unopened. On a regular basis, be sure to slip your underlings a box as a reward for a job well done. It's the classy version of a Starbucks card, especially among men.
Tea. Keep a small box with lots of different kinds. Herbal tea can be a godsend if you're working the 18-hour workday.
Bottle of Advil, antacids, caffeine pills, sleep aids.
Bible. You would be amazed how many people want to share their only passion, and for many people, they're religious.
Memory stick. Get several and give them to bosses along with important files. They'll use them, toss them in their desk and return them by the time they're obsolete. Plus it makes their lives easier, and that's what a good assistant does.
Just some stuff I like to keep on hand. If your boss lets you, get your hands on a junkcatcher. They're a a godsend for keeping track of current papers.