No electronics, no additional instruments. I used to carry a harmonica around, years ago, with the intention of learning to play it, but never used it except as a tuner, or to lend to someone else to play along with me.

So...

Manila folder of songsheets (lyrics)
Tuner
Capo
Picks (one plastic thumbpick, two steel fingerpicks, in small pill bottle)
Spare picks (a thumbpick will occasionally break; fingerpicks are essentially indestructible, but can get lost)
Slide (glass, open at both ends, rarely used these days)
Block of violin rosin

The rosin helps prevent picks from slipping off fingers. When I am playing every day and constantly "in practice," this is not a problem, but if I'm even just a little bit "rusty" and subject to the least bit of nervousness and tension, a finger will slip and its pick will fly off in mid-performance, often landing inside the soundbox, which is an annoyance and interruption, or on the ground/floor, which can be even worse if it can't be readily found.

I scrape the rosin block with each pick, and rub the powder on the picks and fingertips, where they touch each other. Never fails to keep the picks in place.

This is something I learned at Mudcat, of course.

Back in the 60s-70s, my glass slide was a Corecidin (sp?) pill bottle, and I was able to store my fingertips in it. I didn't keep the bottletop, but my thumbpick sorta snapped over the lip of the top opening and served to keep the fingerpicks safely enclosed. During a period of not-playing-very-much, I lost track of the slide/bottle. Now I have a slide that's not a bottle, and a separate little aspirin bottle for the picks.

If you're not a fingerpicker, you may not realize how attached a player can be to a particular set/pair of picks. A considerable amount of time and effort goes into fitting each one to the tip of a particular finger. My middle-finger pick dates back to at least 1972, maybe earlier. I lost its mate in (I think) the late 80s and went through several index-finger picks until settling on the current one sometime late in the last century.