"Composing" sandwiches is an aesthetic and artistic activity (in the catering trade, there are competitions in designing and presenting sandwiches where you can win a gold medal!) A nice one for the end-of-term party, perhaps.
This could lead on to maths, data analysis - bar chart of favourite sandwiches, surveys of most popular bread products.
Bread features in many stories, like Little Red Hen, Hansel amp; Gretel (trail of crusts) and nursery rhymes (eg Tom, Tom, the baker's son). For older children, explore metaphors like "sandwiched between".
Design and technology: Why are some designs more suitable for particular kinds of bread? What breads are useful for what kinds of meal?
Religious education: Bread is a theme in harvest festivals around the world. The Biblical stories of Joseph (going from famine to a land of plenty) and Moses and the Exodus (the Israelites took unleavened bread from Egypt) feature bread, as does Jesus's last supper and communion wafers.
History: Make a timeline from the invention of bread, the first millstones and ovens to sliced bread. What other events intersect the bread timeline?