The sweet filling sometimes used in croissants in France and Spain is almond.
Almond-filled croissants are commonly made with almond cream, almond paste, or frangipane-style mixtures. In France, the resulting pastry is often called a croissant aux amandes. It is frequently prepared from a day-old plain croissant, which is split, soaked or brushed with syrup, filled with almond cream, topped with sliced almonds, and baked again.
This preparation is different from the standard French croissant, which is usually plain and may be served with butter, jam, or other accompaniments. Almond is therefore a recognized variation, not the defining ingredient of every croissant. Apple, custard, and caramel can appear in bakery pastries or creative fillings, but they are not the characteristic answer to this question.
The almond version also connects croissants with the broader European tradition of enriched and filled pastries. In France and Spain, croissants are generally sold without filling, yet almond filling is a documented occasional variation. The contrast between a plain laminated croissant and a richer almond-filled one is a common source of bakery terminology mix-ups.