I'd give anything to see some ancient lifeforms.

In particular, I'd love to travel back to the Cambrian period (~500m years ago) when macroscopic lifeforms began to emerge. There are some real weirdos from this era, such as opabinia (a 5-eyed shrimp with a trunk ending in a pincer) and hallucigenia (a creature so bizarre that for decades paleontologists had no idea how to even orient their fossils).