I thought up a few ways to visualize 2^64 unique items:

- You could give every ant on Earth ~920 unique IDs without any collisions

- You could give unique IDs for every brain neuron for all ~215 million people in Brazil

- The ocean contains about 20 × (2^64) gallons of water (3.5267 × 10^20 gallons total)

- There are between 100-400 billion stars in the Milky Way, so you could assign each star between 46,000,000–184,000,000 unique IDs each

- You could assign ~2.5 unique IDs to each grain of sand on Earth

- If every cell of your body contained a city with 500,000 people each, every "citizen" of your body could have a unique ID without any collisions

Calculating these figures is actually a lot of fun!