A Nano cache is the smallest of geocaches. Its size approximates the tip of a pencil eraser. Typical containers include "blinkies" (see image) and very small pill containers. Because they are so small, swaps are usually not part of these caches.
In many cases there most likely won't be a writing implement so take along a very fine pen. The log will also be very small and so most people usually write only their team name and a date. Any other notes are saved for when the find is logged on the cache page.
They will generally not have any swaps included.