A gift, purchase, etc. whose contents are concealed until after a selection is made.
Winners can choose a gift from the grab bags on the table.
Any random assortment, selection or possibility.
With that system, it's a grab bag which files we can find at any moment.
A moderately large bag of crisps or other snack, intended for sharing.
A bag containing essential items, which can be easily picked up and taken in an emergency