The Romans were certainly better than most in terms of sanitation. They had functioning sewers, public baths, fresh water thanks to the aqueducts, the rich often had internal plumbing! (You can see it in Pompeii!) They could probably use a regular trash service, though they likely didn't produce quite so much garbage as we do today. I guess the most good I could probably do would be to spread more of those advances they already had to more than just the major urban centers and to smaller rural settlements.
Another idea would be to perhaps improve knowledge of some basic stuff such as hand washing and some very basic dental care type stuff that weren't really covered by the commonly held miasma theory of disease at the time.