Squats and deadlifts (and similar compound exercises) are best for building back and core strength. Stretches, especially yoga, can help expand tight chest muscles.

Swimming is good for building endurance and working the whole body.

Note: Don't only work your back if you're asking as a beginner.

Deadlifts will work your entire core better than most other exercises. make sure you research correct form beforehand.

Deadlifts and squats are great workouts for targeting a wide array of muscle groups, including back.

Deadlifts, as mentioned elsewhere in the thread.