Hi
@Alexandra Groszek - I agree with MisMiloKitty and Vivien, I would definitely want to be cat proofing the balcony. A cat can easily jump from a 1st Floor balcony and be gone in a flash. And if Stanley (great name!) were to land awkwardly (which can happen) he could injure a leg or hurt his back. It's not worth the risk.
If you are planning to try harness-and-leash training, I recommend you use only a "cat walking jacket" which IME are the only type that are hard to escape from. (Though one of my cats, a regular Houdini, did once manage to escape from one!

Luckily we were in a friend's garden at the time and I managed to catch my cat before he went far).
These are the two UK companies that make the cat walking jackets:
Happy House Cats :
http://www.happyhousecats.co.uk/cat-walking-jackets-uk/
Mynwood:
https://www.mynwoodcatjackets.co.uk/
(note, both types are made to measure, so they fit well, and if your cat is still growing you will need to replace it as he grows).
Enclosed shared outdoor public play areas for cats would't work, as cats are very territorial creatures who for the most part do not like seeing strange cats in their territory. Taking your cat to an enclosed tennis court where he is the only cat is OK, but as you imply, there is not much for him to explore on a tennis court.
When I lived in a big city, in a 4th floor apartment I used to take my cats to a friend's house. She lived in a quiet part of the city and had a large enclosed garden. My cats used to spend a couple of hours wandering around exploring whilst I followed, holding fast to the leash. The cats loved the garden!
I recommend if you do decide take Stanley outdoors, you should always take a carrier with you, in case of emergencies. It is no fun trying to carry home a wriggling, frightened cat in your arms I can tell you!