Stick to routine consumables that you know you will need like oil to top up, and parts unique to your aircraft (such as an unusual or hard to find magneto). Routine parts that are generally readily available, like oil filters, tires, etc will just reduce your available remaining payload.
I carry needed screwdrivers, spark plug socket, rachet & extension, small socket set & adjustable wrench, small torque wrench, the 1/2" & 7/8" wrenches needed to disconnect a spark plug lead, multimeter, safety wire & pliers, needlenose pliers, a small extendible mirror, flashlight & spare dry cells, small magnetic pickup tool (looks like an extendable pointer) a few spare bulbs, 2 x quarts oil, electrical tape, duct tape, a few wire ties, some extra screws/machine bolts of the correct size for the cowlings and fairings, a bit of wire & wire stripper, an old toothbrush to clean threads, little bottle of anti-seize compound, a couple of small plastic containers with lids to hold cowling screws and such, couple of cloth rags, roll of shop towels, a small container of hand cleaner, plastic window cleaner.
I also carry my cockpit cover, a set of wheel chocks and a lightweight tow bar when I am away from home airport.
