My first unschooling subject, the topic I personally had to learn more about, came up when my son started to walk.
He was putting everything in mouth, so I had to check myself : did I want to mimick the common react "yucky, yucky it's dirty, leave it!"? So I taught myself enough about wild eatable plants (many many more than you'd expect) and toxic ones, and soon we were sharing strange salads from the discarded wealth of nature. "You can eat this one, instead!"
I love globes, I bought us one, and he would quizz me for up to an hour on reading country names, so before 3, he could remember about a hundred of them and became especially intrigued by islands. He started talking much about Greenland, so I told him we'd try to go, but for now, if he wanted to fly, my budget was ok with a few days in Bruxelles. For less than 200€ grand total, my 4 years old son, baby daughter and I flew low-cost to Belgium, got an incredibly generous host from couchsurfing, visited one museum a day. We loved the five senses exhibit, and the DIY scientastic, but not much the kids museum, really made with a 'teaching' mind.
What I find so exhilarating about unschooling is : YES (we're gonna see how to make this happen) is the default answer. Where did unschooling take you so far?
Not teaching does not mean you should not really observe your child and offer. My son was usually running sprints inside our old little flat, and when outside, he was so busy sensing, seeing things that he would walk slowly and stop often. So I got him a trampoline at home, and to share the jumping fun, counted for him - he loved the sound of new words, he was 3. That's how he accidentally learned counting up to 1000 in French and English, and up to 100 in Arabic, Italian, German, Amharic. What uncommon tools did your kids discover for learning math or languages?
One fun consequence is that my son usually starts a conversation with an adult by stuff they might have no clue about, like umami taste, order of the planets in the solar system, purslane season... Or tell me in shock : "you should know everything!"