For the fitness center I like the cardio equipment with touch screen apps like Netflix, Hulu Youtube.  The new consoles are WiFi and you can do everything with them while you exercise.  The virtual reality workouts are cool and the I Fit app is really nice too.  

Dog pool with fire hydrant fountain! 

Over-sized outdoor versions of games (if your demographic is a demographic that would have fun with that). Some would have items that would have to be checked out, but I recently saw a photo posted of outdoor Twister. The spin "thing" was mounted and the gameboard painted on the ground. That might not be the right game for your demographic but I thought that it was a neat idea and there are likely other applications.

Hammock park
I've seen them done all kinds of ways. Various types of hammocks hung throughout the community, actual hammock parks, hammocks in pool area or near playground
Really sell your outdoor space

  
 

Nacho machine and cotton candy machine

Have you considered offering bulk Internet to your residents? I work for a provider and we are seeing more and more conventional properties including gigabit internet as an amenity.