One exercise a day – Mondays are for pushups
Pushups are a great body weight exercise. And you can modify them anyway you like. If you are a beginner, do the pushup off a high surface, like your counter. The better you get at them, the lower your surface should be, until you do them off the floor (whether that is on your knees or toes, is up to you). Pushups should always be a challenge.