If I could only observe and not interact nor tamper with anything:
I would like to see what happened to the Amber Room in Russia during WW2...
I would also like to see be present first hand when the Roswell incident occurred...
If I could interact and tamper with the past:
I would go back to before the Alexandria library burned and read some of the books and bring some back with me.
I would also go back and kill Mohammed before he started Islam.
...
Some very good ones.
No interference:
- go and watch October revolution in 1917 as it happened.
- go and watch Moon landing with Buzz Aldrin and Neil Armstrong.
- sit next to Hubble when he first realized that some stars are outside Milky Way.
- watch Archimedes almost inventing calculus and making Antikythera instrument.
- watch Leonardo painting and inventing, but not dissecting corpses.
- go into future and get some lottery numbers
With interference:
I would go and prevent the fire in the Alexandria's library. It was in the city's port and when Caesar came to capture Pompeii, Caesar's ships were Pompeii's ships in the port with some fireballs. Library close to these ships and it was just in a wrong place in a wrong time.
Tell Nikola Tesla that Marconi is about to steal his patents for radio and stop Tesla from working in Colorado and Wardenclife tower.
I would prevent plundering of Alexander the Great's thumb in Alexandria, by Romans.
I would go and prevent Hitler's mum and dad from making him. The same for Stalin and Saddam Husein
Make a copy of Trump's tax returns and bring it into now. Just for cicks