Today’s all about offerings, both historical and UPG. The Orphic Hymn to Demeter of Eleusis states that the fumigation should be from Storax. There is some debate as to what that particular incense is now called. Some sources indicate that it is known in the modern era as benzoin, so this is what I use for incense when I honor Demeter.
Grain (especially wheat and barley), mint, and poppies are all indicated in the ancient sources as being sacred to Demeter, with barley and mint being specifically mentioned in the drink Demeter asked for in lieu of wine. This drink is mirrored in the sacred drink of the initiates of Eleusis. From that standpoint, it can be inferred that barley is an acceptable offering to Demeter. At the Thesmophoria, pigs are offered to Demeter by being tossed into a chasm and left to rot, with their remains presumably used later to fertilize the fields.
In the modern era, I would say the act of gardening could be considered an offering to Demeter as it is likely the closest we will get to being part of an agrarian society. I’m certain that leaving offerings of home-made bread for the birds would also qualify. Even planting wildflowers to attract Demeter’s bees would be something we could do.