I think I would want to witness the building of some of the really old places, Stone Henge and the huge statues & pyramids created by the Egyptians that they can't figure out how they got there. To think of the amount of people (or aliens if you're a conspiracy theorist) it would've taken and how long it would've taken to build some of those things is mind blowing. It would've taken us years with all our modern equipment like cranes and bulldozers, etc, so how did they do it? I think it would be interesting to find out, and to see it.
First is the Battle of Trafalgar. The battle that made Britain the greatest naval power in the world. It would have been a sight to see as the British ships charged into the Franco-spanish fleet and broke the wall that they made. To see the forest of masts off the coast. The greatest sea battle ever faught.
The second thing I would have loved to have seen would be the wars between the Scottish and te English. I am a romantic at heart when it comes to wars and there are few wars if any that can compare to the Scottish/English wars. The Scottish fighting for their own country against the English tyranny. The wars-like Highlanders clashing with the English red-coated armies or any armies in English history and winning or loosing in epic battles.
At first I considered the Normandy invasion and the crucifixion of Christ, but all that violence might warp me even further. So Im going with the signing of the declaration of independence and the destruction of the Berlin wall