There's also the difference between good secrets and bad secrets. Good secrets feel good to keep, like planning a surprise party or a birthday present. Bad secrets feel bad to keep, like one we are scared about

Also the intention behind a surprise is always to reveal the surprise otherwise what's the point? A secret is something you hide without planning to reveal it generally.

Surprises have an end. Secrets are forever. A surprise is a secret for now, but I can tell you later. Secrets are things I can never tell, which can be dangerous.

I think you need to add in the concept of 'confidental' as well to this. Your address isn't a secret, but you don't tell somone on the internet, even a 'friend' on the internet, where you live.

Oh I love this distinction. I’ve always taught my kid that grown ups should never ask them to keep a secret, (specifically grown ups because part of growing up is having harmless secrets between friends and I didn’t want them to have a complex about that, but at the same time I think it’s important to teach what’s appropriate at a young age for their protection) but I always struggled with how to distinguish a good secret (like a surprise) from a ‘bad’ secret without making things confusing. This sums it up perfectly!

Most surprises start off as secrets.