Hello! I've always had questions about what can be used instead of candles, incense, or other types of fire. Some people have to hide the their beliefs from others and candles and incense are a dead give away. Others, like me, have asthma or allergies or other other health problems that can't be around smoke or strong scents. Or others are concerned about the safety.
Warning: this is long :)
Here are things that have been suggested to me or I've found on the internet:
Crystals and minerals:
Coal, charcoal, lava rock, ashes, wood (burnt or not), petrified wood, amber, fire agate, citrine, carnelian, red jasper, and clear quartz.
Other things (since others might link crystals to witchcraft and be found out):
Anything red or orange (color magic), red or black dirt (especially from lava rock), or again lava rock, ashes, coal, or charcoal, wood (burnt or not), or alcohol (be legal about this, but it could be cooking wine or sherry, or even the alcohol from fermented foods like kimchi or kambucha.)
Oils, herbs, or spices:
Any type of oil really, especially any cooking oil. But if you want to get fancy then these in oil or herb/spice form: cinnamon, ginger, garlic, tumeric, chili, cayenne, black pepper, or any pepper. Hot sauce. No, really, anything spicy-ish. Even soda.
If you have to reeeaallly be secretive:
Make a four strand braid with strings in the colors of the elements you are trying to work with and make a bracelet or anklet or necklace out of it. Take it off and lay it down to work with it.
Or, if you can afford it, purchase or make little charms or beads of the gemstones or crystals for the elements and attach them to a charm bracelet or charm necklace. Unhook the charms when wanting to work with them and hook them back on when finished.
These are like a portable alter disguised as jewelry. Almost like a rosary.
Or! Make a rosary with these colors or stone beads! And put one of the square shaped crosses as the cross so the four legs can be the four directions! (Or, if you are Christian and a witch like me, then it could just be a normal rosary, lol. But whatever, do you.)
But if you have to be extra secretive because relatives or such may think you are wearing a rainbow and they associate rainbow colored things with who they call sinners, try to use more muted colors of string or stone charms. I hope this is not offensive. I am just extra paranoid because I have friends who have had to really hide how they were born because of the beliefs of others are judgemental.
Remember, it's all about what these things represent and symbolize. Think about the functions of things. What do different plants, foods, colors function as. What do they feel like? Cold and soothing. Spicy and burning. So many things can be used.
I hope these are helpful. Please add anything you know of that can help!