iNaturalist★★★★★A citizen science project for reporting and identifying wildlife observations, plus a phone app for use ‘in the field.’ Think of it as Pokémon Go for real animals and plants.
StreetComplete★★★★★StreetComplete is an easy way to contribute to OpenStreetMap on the go, as a beginner, or both from your Android phone.
Vespucci (App)★★★★★Vespucci is a powerful editor for OpenStreetMap on Android that can handle just about anything, though the learning curve is steep.