Indoor rock climbing for sure. A gym will cost you at least $50 a month (where I am, but I doubt it's that much different elsewhere), and the equipment costs aren't astronomical but do add up, especially compared to "cheap" sports (at least $200 for shoes / harness, and additional gear depending on what sort of climbing you want to do).

I'd say even the receptionist at a SV startup can easily afford to do rock climbing if she wants to.

Right, but if they just started at that job, and they previously had worked for much lower pay, they wouldn't be exposed to indoor rock climbing much at all. I don't think the author is saying that they couldn't rock climb if they wanted to, it's just a very different context when people can suddenly have (relatively) expensive hobbies.

The indoor wall where I'm from was at a YMCA, which has financial assistance programs for people who qualify. On a tech salary the author would probably be looking at full price, but if she were still poor it'd be a cheaper option.

At least. NYC is more than double that: $120-130 a month.

In my country the price to do adult soccer is "one soccer ball", many soccer fields are free, you just go and if no one is playing you can use it. The only problem is that you can't play at night because no one is going to turn on the light for you.

The fancier soccer fields with lights on cost 25€/hour, divided by 22 players it becomes just 1€ each ( and you still need to bring your ball )

The gyms in my area are several times more expensive than regular gyms, but as far as a hobby goes it's pretty cheap.

I'd say a hundred bucks a month keeps me in shoes, chalk, and a gym membership

And you know Association football (soccer) is lower class in the UK.