Well I suppose I should consider myself lucky to be alive in both the 20th and 21st century, and in that process two whole millennia.
So much defines our times, that can’t say where to begin. Technology, beliefs, lifestyle, practically everything. Hard to pin point to something in particular. Personally I would like to see how things were across various time periods (and different places at it). The European Renaissance and Industrial Revolution is something I would love to have witnessed.