As hard as they can feel, push-ups are a great move you can do anywhere for upper-body strength.
Start in a plank position with your arms and legs straight, shoulders above your wrists.
Take a breath in, and as you exhale, bend your elbows out to the sides and lower your chest toward the ground. Stop as soon as your shoulders are in line with your elbows. Inhale to straighten the arms. This counts as one rep.
If this is too difficult, do this exercise with your knees on the floor.