Here some example my character carry always on him in form of bracelet, colar, stuck in his air, sand in vial, thing at the bottom of his pockets. He is a collector.
For Fire elemental: something to burn, oil, a small animal thrown in the fire. (Alive if possible
For Water Elemental: some pure water or salt water from a far Ocean, some spit.
For Earth Elemental: rock for afar or gem,gold,silver,iron,sand from a desert.
For Wood Elemental: seeds from a far forest, pine cones, water if in desertic place. Feathers or scales from animals unknow to it.
But for Air Elemental I have no Idea!
Got an idea for a sacrifice to give to an Air Elemental?
Got more idea for the other Element?