I'd take an extra everything if I could afford to.
I've been saying that for years, think I've typed it here too. That's one of the reasons I run a dual amp rig too, one goes on the blink I can make it on the other without touching a thing, just remove the A/B switch. Sometimes I don't even do that, just don't step on it.
I always have extra cables, picks, strings, batteries, even a spare slide bar. Any flaky cables I find out soon after I join a band and they're working perfect as soon as the owners get them to me, and from then on they stay that way. I've soldered mic and guitar cables 15 minutes before a show more than once. Yes, I keep a soldering kit with me too. Extra extension cords, electrical tape, pliers, screwdrivers, crescent wrench, that kind of thing so most last minute gotchas can be dealt with.
I also like to do a practice gig as mentioned earlier, it's great when trying to put together a sound system from two or three peoples' stuff without putting yourself in a bind a half hour before a gig starts. I do that with my guitar rig too, except at low volume. If the amps work at 3 they'll work on 7 too. But amps and pedal board get a sound check at home a couple of days before a gig if it's been several months since all of it has been used. And I've still gotten caught onstage with a pedal suddenly going falky...a dirty jack or something can drop in and toast your cookies just about any time...
One nice thing though, since I started bringing multiple guitars and changing strings every 1 or 2 gigs, depending on whammy bar, I break strings very rarely and always have a standby tuned and ready to go. And can use different tunings a lot more easily.