Depend if it's back-and-forth, or full no-return trip. Because dipping into time to do the tourist-slash-researcher is one thing, but going back to change things, and stay there because of course your original timeline is erased by your actions are two very different things.
The former? Do the tourist thing. Visit original Egypt pre-roman conquest, attend ceremonies to Enlil at Nippur, see a full-blown festival at the Coliseum, chat with Confucius to see if he really had a broom stuck in the...
The latter? Late roman republic. Introduce simple and basic technologies and ideas:
- Movable type printing press
- Immediately after, the academic press, so you can kickstart the scientific method (that lets people figure everything you forgot)
- Accounting & economy methods (so that the gov realizes they can't devolve money without consequences, as they started to do later)
- Germ theory, sanitation/hygiene (there are some nasty plagues coming up soon)
- Simple electricity: show how a telegraph works, let them do the rest ("oh, by the way, yes, it lets you transport energy")
- Simple machinery. Introduce a modern version of the lathe (whose principles were known already), then showcase steam-powered stuff like presses ("I can make a curved shield in 10 minutes").
- Introduce the concept of standardized measures and interchangeable parts for the above.
- A world map. A fuckin' honest to goodness world map, in front of Mr. Julius Gaius Caesar ("you're thinking too small, Jules"). Also, dump Gibbon's Rise and Fall (translated) on him and a few select politicians to see what mistakes Rome needs to avoid.
- And cooking ("No, you do not need to put garum in everything. Try this things called a pizza, instead...")
Then I retire and watch the World Empire start.