As for discoveries, we just celebrated Columbus Day. The United States was not colonized because of evolutionary theory. I believe geography is a science as well.
These continents were colonized over 10,000 years before religious zealots invaded it. It was far more civilized before, with science, international trade, and sustainable food systems.
I second the Hubble Telescope. It's mesmerizing to look billions of years into the past and at the vast unknowns in this expanding universe.
Flight...being able to turn humans into creatures of the air. Helped get Hubble up there after all. Dreamed of flying as a kid and got to really do so in hang gliders for a couple of decades.
Most challenging scientific endeavor...How many human monkeys can we fit on this rock before the chit really hits the fan. Can't wait to see how this works out.