I think it depends on the gig, we have a wedding gig tonight, spare bass (not really a spare as I'll play both), spare class D head, powered pa speakers and monitors gives some redundancy, spare passive Mackie mixer thrown in the van just in case the new digital one goes wrong.
Freebie gig (yes FREE) on Saturday in the band stand in the local park during the day, 45 minute slot, one bass, practice amp.