I thought long and hard about where and what I would like to see. I thought about adventures in evolution (I would love to see the Burgess Shale critters in action - talk about living sci-fi), and some of the more renowned geologic features in geologic time (I would love to be a fly on [or in] the wall to see the onset of the San Andreas strike-slip movement or to watch the very young San Jacinto fault (~700,000 years) from start to present in fast-forward).
What I would really love to see are two water-related events from Quaternary time. The first of these are the Missoula floods originating in western Montana from the repeated failures of ice dams on the Clark Fork River. The second are the Quaternary lakes of the Mojave desert.
The repeated Missoula floods (potentially 40+, from the varve-like slackwater deposits in southeastern Washington called the Touchet beds) carved out the Channeled Scablands of eastern Washington. The lakes, coulees, and river channels of the scablands were carved by the massive floodwaters (up to 500 cubic miles) that occurred repeatedly through the Pleistocene. They were identified in the 1920's by Harlan Bretz, who was derided as the flood theory of scablands creation just didn't mesh with the theory of uniformitarianism. Bretz's theories were finally accepted after years of research by himself and others, and I believe he was awarded the Penrose Medal for it. The flood events are also called the Bretz floods in his honor. The remaining lakes provide refuge for migratory birds, riparian and wetland habitat, and recreation opportunities for locals and visitors. I spent my childhood driving through eastern Washington passing by little pothole lakes, large, wide valleys with little ribbon creeks and riparian in the middle, scraped basalt ridges, and the windblown loess hills of the Palouse, wondering how on earth they got there. Luckily, between my geotech/engineer dad and local natural history class in junior high, I got to understand why way before college.
While living in SoCal, I spent lots of time in the Mojave, especially going up to Owens Valley or over to Panamint and Death Valleys. I occasionally went to Saline Valley and twice, maybe, to Deep Springs Valley. All of these valleys contained pluvial lakes, the largest being Lake Manly. The Mojave River flowed (just like in 2005) from the San Bernardino Mtns into a string of lakes ending in Lake Mojave. [I vaguely recollect that Lake Mojave may have had a discharge into Lake Manly via the Amargosa River]. I spent a very long day in 2005 driving a circle around Death Valley with my mom and 11-month-old as I couldn't get a hotel room. That year was probably the only time in my lifetime I would see Lake Mojave with water, the Amargosa River (in the distance) flowing, and Death Valley Lake a lake. It wonderfully blew my mind, and I would love to have seen all the rest of them full, as well. A few pictures from 2005 are below.
Silver Lake (filled from Lake Mojave/Mojave River)
Death Valley Lake