Seek by iNaturalist (Free!)
Seedling Scavenger Bingo by Seedling (free, in-app purchase to do DIY hunts)
Silly Street Learn & Play by Silly Street (free, in-app purchases to buy more card packs)
Drawnimal by Yatatoy (paid)
Storest by Pixle (paid)
Honorable mentions (there's so many good ones!):
Color Clues by Jesse Foote (free, another type of scavenger hunt)
Elmer’s Photo Patchwork by Touchpress (paid, decorate Elmer with a photo collage)
Hey Clay by Hey Clay (freemium, learn how to make clay sculptures)
Novel Effect by Novel Effect (free, provides movie-like sound effects while YOU read books aloud)
Foldify by Pixle (paid, design 3D figures on the app, then print, cut, fold and glue to make 3D paper toys)
Pinna by Pinna (subscription app for kids' podcasts)