Keep spare candles to represent the elements you cast circle with, whether it's fire, earth, water, air, metal, wood, or spirit. It's always good to have some spare.
Keep at least one spare candle of each colour, but have a couple extra white ones because you can infuse these with runes or tie a coloured ribbon around them to show the colour you're using.
String and ribbons can be used in spells to create chords, or to symbolically express connection or release. They can also double up to decorate candles.
SPOON KNIFE TWEEZERS
It seems odd, but this is a good idea: a spoon to help mix potions, a knife if you want to carve into candles, and tweezers in case you drop something small or your wick is trapped by candle wax and you want to get it out without making too much mess.
Mirrors are used in quite a few spells and it's always good to have one spare.
HEAT PROOF DISHES
For candles and burning things.
Of course, you can substitute this with a lighter, but I prefer matches.
SMALL FIRST AID KIT
It has nothing to do with spell casting, but in case you get burnt or cut whilst you're casting a spell you want to be able to sort yourself out quickly. If you don't want too much fuss, just have a box of plasters, antiseptic wipes, and an instant cold pack.
BOTTLES AND VIAL GLASSES
In case you break something you're about to fill with a potion or tincture, make sure you have some spare.
It's always good to have a spare smudge stick.
Salt is good for everything. Everybody needs salt.
There are many other things you may want to add to your own box, but I feel these are most important.