I am allergic to alot of incense so I sometimes use different things that correspond to the element that needs substituting (also useful if you are in a witchcraft intolerant household).
Altar: small scarf or square of fabric.
Air/Incense: Oil burner: feather, small bird statue, little toy aeroplane, a clear or pale crystal (clear quartz, angelite).
Water: Shell, small piece of coral, toy sea creature (fish, crab, lobster, sea horse, starfish), submarine or boat, picture of the sea, small pebble from the sea shore (if you can one), a blue crystal (I like blue lace agate as it has wave-like markings on it).
Fire: Some ashes from a fire, a birthday cake candle (never lit), a dried chilli (the hottest you can find), a picture of fire or a volcano, a fire-tone crystal (red, yellow, orange, like ruby, carnelian, orange calcite, citrine) or an obsidian crystal (it is volcanic glass).
Earth: Some soil, a small stone, some salt, a leaf or flower, a tree seed (acorn, conker, pine cone, etc), a picture of a tree or mountain, a brown or green crystal (green moss agate, any earth-tone agate, emerald, jade).
To represent the Goddess and God it could be something like a small statue of them (if you have a particular pantheon that you worship) or something simple like a drawing of the sun and moon, or even one gold and one silver ring or bracelet charm.
This can all be kept wrapped in the scarf (apart from the oil burner, ) and placed in a drawer or small box, and if anyone finds it they will just have a jumble of little things that make no sense to them.
