Maybe you just have advantage on saves against any effect that can inflict the unconscious condition (so Eyebite, Drow Poison, Symbol, traps, etc.) so long as you’ve consumed coffee since your last long rest. You could even impose disadvantage on those same saves on any days when you don’t drink it to reflect grogginess and/or withdrawl.

It’s a minor passive benefit most of the time, and it means tracking your coffee supply like rations and giving you an exotic good to spend some of your gold on whenever you’re in town, but it’s not super cumbersome or anything. Plus, Eyebite is one of the spells that WotC actually puts on default monster spell lists somewhat frequently for evil casters, so having advantage on saves against it might actually come up