Enrichment for a day gecko is environmental rather than something you hand them. Unlike a dog with a toy, a Phelsuma grandis expresses its natural behavior by climbing, basking, stalking prey, and exploring a complex three-dimensional space. A well-built terrarium is itself the enrichment, and the goal is to make that space rich enough that the gecko has reasons to move and forage throughout the day.
The behaviors worth designing for are climbing, hunting, basking, and retreating. A layout that lets the animal choose among warm and cool zones, open perches and hidden refuges, and surfaces at varied heights gives it the daily decisions that keep a captive reptile mentally engaged.
Enrichment also reduces problems. Geckos in barren enclosures are more prone to stress, glass-surfing, and lethargy, while a varied environment encourages the natural activity patterns that keep the animal fit and its appetite healthy.