A book of mine suggests stacking them using different coloured fillings, and serving by cutting triangular pieces like it's a cake, with a sharp knife. Some suggested fillings were:
1. Smoked salmon with freshly ground black pepper or squeeze of lemon
2. Avocado mashed with lemon, salt and pepper
3. Tuna mixed with a bit of mayo, S & P
4. Egg mashed with mayo
5. Asparagus sliced in half
6. Sardines mashed with lemon
7. Cream cheese with chives
8. Grated cheddar cheese
9. Slices of edam cheese with light chutney coating
10. Ripe Brie or Camembert
11. Hard boiled eggs sliced and sprinkled with paprika
12. Chopped eggs with cream, salt and pepper
13. Cottage cheese mixed with cream till smooth and seasoned.