For modern fitness sites, consider Joomla 5 templates like "GymGuide" or "FitPro" from active developers (e.g., JoomShaper, Youjoomla).
The most critical feature for any fitness site is the class schedule. Members need to know when Zumba, CrossFit, or Spin class happens. JoomlaXTC - Fitness Life for Joomla 2.5