Collect the listed number of Dragon Eggs to unlock the listed option in the "Secrets" menu:

Concept Art 1
Character Viewer At least 11
Ember Model At least 21
Flame Model At least 31
Sgt. Byrd Mini-Game At least 41
Spyro Turret Mini-Game At least 51
Sparx Shooter Mini-Game At least 61
Blink Mini-Game At least 71