 Anything you can imagine! Some suggestions that readers have made in the past include: chopped up olives, anchovies, pickles, a small meatball, cooked chicken, sausage meat, chopped up Spam, corned beef... as long as it's not too wet, compact and a bit salty, anything can be a good onigiri filling.  Anything you can imagine! Some suggestions that readers have made in the past include: chopped up olives, anchovies, pickles, a small meatball, cooked chicken, sausage meat, chopped up Spam, corned beef... as long as it's not too wet, compact and a bit salty, anything can be a good onigiri filling. 