Flippant answers aside for once.
Attention. Respect. Engagement.
And a small group people with whom you could easily see sharing a 3 to 4 hour car trip, enjoyably.
Everything else is a bonus.
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Now if I was going to go back to the original question and talk about essential TOOLS, I'd talk about Roll20 (which I use even during some offline game sessions), Dropbox, et cetera.