Keep in mind that anything that is flammable normally is going to be flammable in your candle. Adding things that are tiny like spices such as cinnamon may clog the wick and may interfere with the candle burning.
Cinnamon sticks can serve as a secondary wick. Often people add embellishments of real flowers, roses, or crystals. You can add larger herbs.
Candles of intention are made with a combination of fragrances, herbs, stones, crystals, and florals with purpose. These are meant for rituals and spells.
Another topper can be to add wax figures made from small silicone molds. You could adorn your candle with small charms or dust the top slightly with a little mica to give it a shimmer. Remember less is more. You are already creating something that burns, don’t create a fire starter!