If all you're having is borei nefashot foods, it's probably not necessary to do anything different. Say the bracha rishonah quietly before you take a bite, and a borei nefashot at the end. It's not that long.
If you have to make an al hamichya, i would just tell them, "i'll be with you in a second, i just have to say a short grace after eating." As Double AA mentioned in a comment, many gentiles are familiar with the concept.
And if you're having bread and need to wash and bentch, treat it as a teaching opportunity. Explain to them: "Before eating bread, Jews do a ritual handwashing and say a blessing. When we finish eating, we say grace after the meal, in Hebrew." That should alleviate any odd looks, assuming you're eating with a civilized person. They may even ask for more information, or to hear you say it out loud. :)
Basically, don't worry about it.