A gouter is the snack that all children in France have at 4pm. Very popular is a chunk of baguette with a bar of chocolate in the middle! Believe it or not this is a very popular afternoon snack among French children.
If that seems excessive, get hold of a jar of Nutella - chocolate spread - as a good alternative. Every single person in France is familiar with Nutella - as their publicty says, '100% of French people have tasted, do taste or will taste nutella'.
Alternatives include a cake from the local delicatessen, a biscuit, even fruit if you have compliant children.
As far as I can tell there is an alarm clock built in to the heads of French children, that rings at 4pm to remind them it is time for a gouter. Any other meal can be delayed indefinitely with no consequence but the danger of missing a gouter is simply not to be considered.
More of a light lunch than a gouter perhaps, but if you have time on your hands you might enjoy a croque monsieur in the middle of the afternoon as well.
Butter two pieces of normal, sliced, bread (yes, this is available in French supermarkets)
Place a slice of ham cut to the size of the slice of bread, and some semi-hard cheese, on top of the first slice of bread (often gruyere cheese is used)
Put the other piece of buttered bread on the top, squeeze it together, and put in the oven for 10 minutes until the cheese has melted and the bread is toast.
Common variations include adding dijon mustard, or brushing melted butter on the outside before cooking. Sometimes grated cheese is sprinkled on the top as well. In truth there are as many variations as there are cooks.