I've thought along these lines, too--a playground for adults. The major problem is, children play because they see other children playing, want to have fun, so they get involved in 'the game' (whatever that might be). Adults are generally more inhibited and need 'a reason' to 'play'. They tend to be more 'goal' oriented--which is why sports are so popular.
There are, already, for example, indoor climbing schools:
Indoor trampoline parks:
Indoor freerunning (parkour) courses:
Indoor gymnastics courses:
Indoor "adventure" parks:
And so on…
So my first question is, what is it you don't like about a Gym, Being? What is it you're looking for but can't find?
I've been thinking that our technology is at a point now where 'someone' somewhere ought to be able to come up with some kind of technologically advanced 'work out' system that would revolutionize the whole concept of 'resistance training'. For all practical purposes, we're still just 'lifting rocks' (cf. stone "halters" used in ancient Greece
I'm imagining 'something' where the whole body, and its various ranges of motion, is dynamically contained. Rather than listening to our favorite tunes on our iPod, we're 'immersed' in a fully virtual experience that challenges us physically based on our fitness goals and our *psychological* profile. No more 'video games' where all we twiddle is our 'joy sticks'; this is a total body, mind, immersive experience. The programming could be anything that would get our hearts pumping, adrenaline rushing, and muscles moving. It could be anything from a more or less 'traditional' workout scheme (resistance [weight], reps/sets/temp) accompanied by whatever 'motivates' us to go beyond our limits; to full-on virtual competitions and battles.
The system itself will record and monitor our progress toward our fitness goals, advise us what to eat when and make it possible for just about anyone who wants it to get a 'Superbody'.
I wouldn't be the least bit surprised if some military contractor somewhere is already RnDing something along these lines--although it could be another century before it is available (probably after the digital/neural interfacing gap has been crossed).