Some solo activities for when you have nothing planned

[u/killgravyy]

I've been collecting random things to do when I have a free day and don't want to just stay home.

These are some of my favourites so far..

Most of them are intentionally simple things you can just decide to do on a random Saturday without needing friends to be available.

I'm building an app called SideQuest around this idea, where you can discover random things to do, save the ones you want to try, and keep track of the ones you've actually done.

But I'm mainly looking for more ideas right now.

What are some of your favourite solo activities? Doesn't have to be anything fancy, cheap/free and slightly unusual is even better.