Find out what sports he plays, and brush up on the lingo. This way you can chat with him about his favorite sports, and impress your new athletic guy.
Work out at his gym. This can be a bit harder, but even if you are new to a work out routine, it can't hurt. Start slow, and if he is around, ask him for pointers on which weight machines to use.
Be confident and sure of yourself around him. Being an insecure gal is always a turn off to any guy, athletic or not.
Ask him to go to the park for roller blading or disc golf. It is always easy to impress a guy when you have a plan. It doesn't have to be a big deal, so just ask him casually if he would like to join you and some friends for a nice weekend park time.
Impress an athletic guy by going to his sporting events. If he plays for a school team or has a team he plays with outside of work, go an watch him some time and cheer him on.
Take care of yourself and be healthier. An athletic guy is most likely interested in taking care of his body, so you need to hide your Twinkies when he is around.