I always carry a back-up 7 4pc 6wt Fenwick glass rod which I sling over my shoulder or leave in the car.
I always carry a spare spool and line.
I always carry 20lb Maxima (fly line to leader)
I always carry extra tippets and extra hand tied leaders in zip lock baggies.
I always carry extra beadhead nymphs and woolly buggers(even for saltwater)
I always carry a light weight rain jacket and down vest.
Randy