some good options already posted:

the pyramids being built

the big bang

to that i'll add:

watching a t-rex and seeing what that thing actually looked like and how it hunted (this one is kind of stretching the rules a bit)

Jesus Christ when he was alive in his prime years. I want to see him turn water into wine. This would answer if he was actually real and if he really did that. (spoiler I believe he did but it would be interesting to see).

I would love to see the building of Machu Picchu

This is all assuming that I am able to view without affecting history in any way shape or form and am able to survive the experience (with my sanity intact)