Little hardware items like wheel collars, clevis, screws, fuel line, glow plugs, zip-ties, rubber bands, etc. Nothing is worse than not being able to fly because of a broken or missing 25 cent piece of hardware. I also have CA glue, 5-minute epoxy, packing tape and other repair items just in case. That's all in addition to basic tools like screwdrivers, allen wrenches, knife or razor blades, crescent wrench and so on.