Any recommendation for free/cheap things to do in Stockholm in late May?

I'll be in Stockholm from 25/5-28/5. Other than walking around places (of which I have an extensive list), does anyone know of things to do that are free or cheap (<180 SEK)? I am especially interested in history, art, and nature but am really willing to do anything. I am by myself.

I know there used to be a lot of free museums, but I think those have fallen out of fashion as I haven't been able to find any. If something is more pricey but worth it (like day-long activities/trips), I'd be open to it.

I have a UK student ID and I know some places offer student discounts, but from what I've been able to discern they're just for Swedish students.

Thank you!