To work on arm strength you might try 90 degrees drill- hip drill: Use a 4 seam grip. Have a partner about 25 feet apart. Mark off your stride line. With your hands at break point pull the ball down and out out of your glove. To gain momentum shift your weight from back leg to front leg. Point your front shoulder and elbow at your target and throw finish with a good follow through. This drill works on arm stretch and strength.
This is also a drill that pitchers use that is of benefit to outfielders as well. The difference is that you will use a 4 seam grip. Kneel on your drive leg knee with your stride leg bent towards your target. Make sure that your stride foot is in good position. Have the bulk of your weight over your back leg. Rotate your upper body so that your throwing arm gets into the cocked position. Focus on the target at this point. As you make your throw, transfer your weight from the back leg to the front leg by pushing off the back foot, this will take you from a kneeling to a standing position. This will make you focus on the hip rotation and arm extension and follow through. Have your target about 20 feet from you. This again helps develop strength and stretch.
Flintoide has some good drills for outside space for the most part. If your space is limited then use angled ground and fly balls to concentrate on your drop step and crossover. Overhead balls will help you to open to the glove side. If you have additional outfielders then you might add communication drills to help coordinate footwork and emphasize back up responsibilities and develop leadership skill especially as a centerfielder. Good luck this season.