The push-up. It's a classic calisthenic exercise with so many benefits! It works your arms, chest, and core like no other bodyweight move. And working your pecs, the muscle directly under your breasts, gives the area a bit of lift. And who doesn't want to fight effects of gravity? You can change your hand positions to focus the work on different muscles, but if you want to truly challenge your pecs, try elevating your feet.
I like to put my feet on a bench, or sturdy chair, that puts my shoulders and hips in one line, perpendicular to the floor. The higher your feet, the harder the push-up.
This variation is challenging, but push yourself to try to do as many elevated push-ups in a set as you do with your feet flat on the floor. But in pushing yourself, do not sacrifice your form. If you start sagging in your low back, take a break.