For quartet gigs:

pitch pipe,
water,
set/show list of song titles showing what key and first couple of words,
address of venue.

One of the others usually has gum, another has the large-print lyrics handouts we use during hour-long shows at retirement residences.

When I first joined the quartet I also brought the whole assortment of sheet music in case I needed to check on something while warming up, but we don't use sheet music or lyrics 'on stage'. More than once someone has made up words to rhyme with my mistake!

I barely have time to keep basic instrument skills alive, never mind performance-ready. But I keep the usual pliers, spare strings, capo & picks in my guitar & banjo cases; rosin & spare strings in cello case. Tuning fork in guitar case or pocket.