So somebody already mentioned the feedstation portables book and rice cakes. That works really well.
Marzipan works, you don’t need to use egg whites, I’ve had success with just using water and a tiny amount of honey to bind it together.
Sandwiches.
Pork pies have seen me through many a long ride.
Nice potatoes – microwaved the night before, small amount of strong cheese in the middle. Fantastic savory snack and easy to carry and eat.
Potato cakes. Make a sandwich using a bit of cream cheese to stop them being dry. Cut in half is about right to eat in one go.
Flapjack – experiement as my friend makes a ginger one which is amazing.
Spanish Omelets.
There is a good video of how to wrap food using foil parchment. Creates a robust but easy to open package which makes some of the more unusual snacks easier to handle.