For me it's mostly tools. Spare belts sure. Some hoses are next to toss in a bin. Having a place to put all the tools AND spare parts is a problem. Considering I haul 3 tonnes of armor and all my re-enacting kit when I go.
Off the top of my head:
2 gallons of oil
2 gallons anti freeze
4 jerry cans
spray lube
lithium spray lube
wire brushes
air tool set (impact, die grinder, chisel, grease gun)
robo Impact tool
20 ton bottle jack
pair of 6 ton jack stands
wood blocking
two sets of chocks
triangles
20 lb fire extinguisher
1/4, 1/2 and 3/4 socket sets
limited 1" socket set (and breaker bar)
pioneer tool set
small sledge hammer
pall peen hammer
punch/drift and chisel set
inch combination wrench set plus the larger 5 wrench set Sears sells
air compressor wrench
grease
spare brake fluid (I need a larger container of this)
x3 5 ton snatch blocks
100 lbs of various chains and binders (for the load and problems)
spare set of shackles for anchoring chains
2 spare mounted tires plus standard spare (last trip had 3 spares)
Air inflater tools
25 foot day glow orange air hose
x2 yellow coiled air hoses
2 flashlights (lantern battery type and a crookneck)
battery powered spot lamp
spare jacket
spare sweater
spare hat
I need to add a first aid kit
TMs in printed form
wire bullets and crimper
insulating tape
zip ties
spare wire
thread tape
brake parts spray cleaner
electronics parts spray cleaner
Umm, I think that's it.
[edit]
oh, googles
work gloves
ear protectors and some spare plugs
razor blades
and some permatex though I think that tube needs to be replaced.