Hi
Whilst it is not possible to delete the Activity app, Workout app or Breathe app on your Apple Watch, the fitness tracking features can be disabled:
On your iPhone, in the Watch app, go to: My Watch (tab) > Privacy > Motion & Fitness > turn off Fitness Tracking and, if desired, also Heart Rate.
On your iPhone, in the Watch app, go to: My Watch > Activity: turn off all options, including setting Progress Updates to None.
On your iPhone, in the Watch app, go to: My Watch > Breathe: set Breathe Reminders to None and turn off Weekly Summary.