An offering is just an item. It is preferred, although not necessary, that the offering be a natural or earthly object, or made from natural items. These could include, but are not limited to herbs or flowers, incense, oils, crystals, stones, are anything you feel is symbolic to either your deity or even yourself. 
 
This copper offering bowl would make a lovely addition to your altar, and given the size would be perfect for a travel kit.
Measuring 7.5cm x 10cm