Always good to have a spare fan belt , spark plugs & leads , rotor arm , dizzy cap ( depending on you vehicle) 1 1/2" -2" hose (depending on your car) and a few cir-clips jerry can of fuel and enough water and coolant to charge the system IMHO . tyre weld in a can .
Wont take up much room in your trunk but it'd sure speed up the repairs if you have issues bigS . I tend to have 2 spare wipers in the boot too as when they go wrong it always seems to be in the middle of a rain storm .
I tend to keep the old parts after a service as it can still be a useable spare .