Suppose you have a space-time machine to (safely and comfortably) watch an event unfold; which event would you most like to see? Why? What do we already know or hypothesize about that event that appeals to you, or that you would like to test? What would be the result, the upshot, of knowing more about this event? You do not necessarily need to limit yourself to Earth, nor to the past. You do not need to limit yourself to a particular instant if peeking several times over a period of minutes or ages helps you envision the evolution of something. You do not need to limit yourself to environments that could support life as we know it... imagine being able to take a time-sampling of magmatic composition from 10 miles below the surface as a nascent mid-ocean ridge opens up, or examining the circumference of the vent during one of Yellowstone's mega-eruptions! I'll tell you, this technology is basically magic. (See the third law here.) Feel free to toss in a few "also-rans" of your favorite day-dreams, but please develop one. This isn't intended to be just a set of lists.
Anyone? If you'd like to play along, please either leave a comment here or post in your own blog and leave me a link.