You'd be surprised by some of the simplest things that were invented that people literally had right in front of them, penicillin comes to mind, which grows on decaying organic matter, for example, the bread in your house can have natural penicillin in it, it's likely that humans have been using penicillin for millenia for infections in some form and never knew about it or how it worked.
Another is yeast when it comes to cooking, also extremely common in the past but soft bread was only discovered fairly recently by historical standards, heck some of the spices we use today were considered poisonous in the past.
Probably the biggest examples though is oil and turbine technology, oil was considered a nuisance for thousands of years, literally seeping out of the ground and being considered a problem, now it drives our entire economy, turbines are a even greater example in some ways, we came VERY close to relying on steam power (though tbh we still kinda do, just more reactive means), we have had access to the basis of turbines (watermills) since the BC era, but we didn't know they could be used for electricity until i believe around the late 1700's, now the best source of electricity, nuclear power is basically a giant water boiler that pushes a turbine.
Truth is most of our root innovations we came across by either chance or by throwing stuff at a wall until something stuck.