The most common things people quantify include:
Heart rate variability — your heart rate, stress, energy and productivity levels
Activity — number of steps, speed and distance
Productivity — amount of time spent on social media, reading the news, and procrastinating.
Workouts — goals & progress, calories burned, distance and heart rate
Sleep — number of hours, sleep quality and stages, temperature, noise levels in the bedroom, and more.
Meditations — time spent meditating, average heart rate during sessions
Nutrition — calories consumed, nutrient balance, caffeine and sugar intake
Health — biomarkers such as blood pressure, blood sugar, heart rate, blood oxygen levels, weight, BMI, etc.
Environment — air quality, temperature, carbon dioxide levels, humidity, etc.