Well...
They had windmills already, IIRC, so I can leave that out. EDIT: Apparently they didn't aside from a prototype by Hero of Alexandria. I don't know how much windmills could do for the Romans that waterwheels couldn't, though.
A bicycle would be nice, but unless we're giving them the tech to make a modern bicycle, there's a reason nobody used them in Roman times and it wasn't that they were too dumb to make one: it's just a worse wheelbarrow until you get some pretty good precision work. It needs quality roads too, but the Romans had those.
Gunpowder is a good idea, and would be my second choice: it's certainly doable by Romans too (heck, it could've predated the wheel if folks had gotten lucky mixing random chemicals).
The microscope isn't all that useful without a system of scientific inquiry around it, IMO: there'd be a lot of neat engravings/drawings of close-ups of various things, but it wouldn't lead to germ theory or similar things in time to save the (Western) Roman Empire.
So I'm going to go with the printing press, which will increase administrative efficiency and cultural propaganda, and is harder for the "barbarians" to duplicate than gunpowder weapons, which every German tribe and Persian army will have in a century or so if the Romans get them.