Foam Roller or Massage Balls
I don't know how we got along without these simple tools. They are great before and after practices and meets to create more supple and pliable tissue. Also great to use after a long day of travel. If you have been rolling out with foam rollers or balls for a long time, the next step up is a lacrosse ball---which fit well in a stocking!
Watch or Gym Boss Timer
Everyone needs to be doing more interval training and you can't have an effective session without timing both the work interval and recovery. Look for features in a watch whose timer has the option to do both count down timing and continuous count down timing. The Gym Boss is great because is has the ability to set the number of reps and easily accommodates different work to rest times with it's dual countdown timers.
Athletic tape is to an athlete as Duct tape is to a handyman. There are hundreds of ways an athlete can use this tape; marking a steps on the runway, setting a go zone for a relay, taping shoes, name tag for bags, identifying water bottles, etc. Don't go to a track meet without it.
Everyone needs a hoody. The perfect workout gear. The one shown below is from Elite FTS and reminds you that athletes TRAIN, and don't workout.