For me it was doing more physical stuff.
A class came up on Facebook and my first thought was ooh I'd love to do that and my second thought was dismissive and that that sort of activity was not for someone like me. Somehow I caught the thought and booked on to the activity anyway. I'm getting used to catching those limiting thoughts/beliefs and for me, the only way out is through.
So I had a beginner session at a water sport, specifically it was aimed at women so felt easier. Both going there to do something totally different (but cool... And I thought I couldn't/shouldn't/wasn't good enough/ who did I even think I was! to do cool stuff) and putting myself in a situation where I'd know no one were out of my comfort zone.
So for me, sea dipping which someone has already mentioned, various water sports, hiking, I learnt to wild camp and what I'd need as the bare minimum to keep myself warm, safe, dry and fed, I've done extended outdoor first aid courses so that I could feel more competent as part of a team if anyone needed help.
It's totally changed my view of myself and I feel so much more competent. I've stretched my comfort zone so that I can say yes to even more things...
I don't know if that helps. I imagine many people will do those sorts of things without a second thought but they might feel terrified doing something academic, or creative. So for them taking a painting class for example or a philosophy course or whatever might be a massive deal but eventually they'll see themselves not as someone who would like to have a go at painting but daren't, but as a painter.