Note: My answer isn't about the highest-impact thing you could do. I also don't face (EA-related) guilt/anxiety about downtime because I repeatedly remind myself that I need not to burnout.

I reserve an hour each night as 'transition time' between work and downtime, with half of that hour being dedicated either to therapy (which is important in sustaining myself so I can solve important problems) or reading tabs (so I can prioritize them and not waste 'work time').

The latter might be more relevant here. I use Notion to clip tabs, such as Forum posts, that seem interesting and categorize + prioritize new tabs at the end of the night. Then I start going down my reading list until I reach the end of my allotted transition time. This both saves precious non-dead time and seems like a productive, EA-related use of dead time (in that I often learn more about EA during this time).

I think I gained a lot from reading blogs and writing comments on the internet, but of course it is very costly in terms of time. I think I'm happy with the benefits, but I have a poor understanding of the counterfactual (like if I spent the time reading books or doing small programming projects or something else instead). Also to be clear I mostly started writing internet comments because it was the natural thing to do, and not because I had any pretensions that it'd be useful. I would be very surprised if commenting on the internet is anywhere close to optimal, as my initial reasons for doing so are far from virtuous. 

In addition, my understanding/guess is that internet engagement is substantially more emotionally/cognitively costly to other people than for me. Still, you may find it an attractive option until you have better ones.

If you find commenting on the Forum or LW during times when you're very tired/low-energy unappealing or scary, consider commenting in other communities with reasonably high epistemic quality (eg analytic blogs about your day job, Q&A sites like StackExchange, strategy articles if you play a competitive game).

I've also heard good things about podcasts like 80k, though I have not gained as much from them myself relative to (inter)active participation. I've enjoyed the relatively few documentaries I've finished watching though I have not found them to be too useful.

Forecasting on platforms like Metaculus and GJO is another option. 

I have a long (1-1.5 hour) commute via public transit to and from my work and as much of a time sink it is, it serves as (1) a great environment for doing offline non-professional work and (2) a concrete divider between my "professional" part of the day and my "personal" part of the day.

 Typically, I do a mix of (sometimes EA-related) things, such as listening to downloaded podcasts on my AntennaPod, reading articles tagged as "priority:high" on my Wallabag read-it-later app, listening to books on Libby, or watching non-intensive, career-related MOOC lectures via Lynda (now LinkedIn Learning), UDemy, etc. Admittedly though, I've struggled with time management as of late and have often have to use that time to sleep instead (burnout is real!).