You must have only read the first one or two Danny Dunns - a girl, Irene, shows up very early in the series, and is an equal to the boys, except possibly smarter and definitely possessing a lot more common sense. I'd actually forgotten that she wasn't around from the get-go until I read that early one last night.

I don't know if you would have liked them anyway, though. They're about exploring strange new worlds, not fighting bad guys - "Oh no! Can we ever escape the bottom of the sea?!" rather than "Fight the villain at the bottom of the sea!"

I read the later (written by someone else) Tom Swifts, which added two kick-ass girls and actually are about fighting aliens/robots/etc.