A good multi tool
Particularly while racing, I like to keep a good multi tool handy. The one I’ve found to be the most useful is the Leatherman Skeletool CX. It’s got a nifty clip built into the handle, so it can clip on to your belt loop.
This tool is lighter than most others, (the CX model has carbon fiber and Titanium coating and is very corrosion resistant) it has a really sharp knife with serrated edge, and a minimum of tools, but all you really need. It’s not cheap though, the best never are. You can sometimes find them used on Ebay, but not at that much of a discount than this new one. This is the best price I’ve seen.
Again, best features are light weight, belt clip, and a very sharp knife that stays sharp.