Half a banana. The original emergency food. Bland, gentle, easy to eat, easy to keep down.
Bananas are part of the BRAT diet (bananas, rice, applesauce, toast) recommended for nausea. Half is fine — wrap the rest in cling film for later.
Slightly underripe bananas (a little green) are less sweet and can be easier on queasy stomachs. Frozen banana slices are also soothing.