From my 20+ years experience, I can tell you that the watchtower certainly discourages these activities, especially if they would be done in association with "worldly people" as in school after-hours athletic pursuits or commercial training facilities. They'd (the WTS) probably come down hardest on martial arts, believing that such training is "preparation for war." I think that at one time a watchtower publication actually said that self-defense was an innate knowledge and did not have to be learned or practiced.
(Hypocrasy, when you learn that the WTS officers have BODYGUARDS!)
Now, in actual application, the rules flex tremendously depending on local conditions - and "local" can be down to the level of who your parents are. I knew some guys who were in my age group growing up as JWs who were both fairly athletic, practiced judo, and served in their restricted-entry high schools as hall monitors, engaging in all the juvenile conflicts that come with that territory. It was OK for them - both Mom and Dad were Dubs with "rank" in the congregation, and they could do no wrong. Don't get me wrong - these were both good guys, and I envied them for the freedom they enjoyed that was denied me.
I think that in dub-dom, if you ignore their little rules long enough to become GOOD at whatever it is you shouldn't be spending time on, you get a free pass - a nod and a wink and let's move on.
So, in my experience, formal practice of martial arts was out of the question. If you went to the beach and engaged in some romping with the guys, well, that's just horseplay, right? Working out with weights was likewise discouraged - "bodily trainng is beneficial for a little, but godly devotion... blahblahblah." Usually if you started to workout you would begin to hear negative comments. Still, there were exceptions. It was OK for someone in the circuit to donate an Olympic weight set for the use of the bethelite who were assigned to live at the assembly hall. The guys who availed themselves of this opportunity got pretty big pretty fast - ah, but they were of a different caste than mere publishers.
School sports, as i said, were discouraged because of the 'bad associations" they would involve.
By the way, Chess was practice for war also - strategy, you know.
In one way, these restrictions serve a useful purpose - they get the kids to abandon the religion early on.