Nuts have a lot of calories in a small package, and as a migraneuse I can also verify that nuts and dried cherries are one of the least offensive things to deal with if you end up puking. If you have miso, miso soup is easy and gentle and tasty.
But mostly make sure you are drinking water with electrolytes. Your body can survive well without food calories for quite some time, and lack of appetite when sick may be a signal to fast and rest. Dehydration is much more likely and much more damaging.