earlier crop rotation is another cool idea
You had a form of crop rotation, bi-annual, practiced in Roman agriculture and that remained in place roughly until the agricultural revolution of the XVIIIth-XIXth centuries in southern Europe (in its larger sense). As with other medieval agricultural devellopment (such as the heavy plough), it really had an impact (and a really important one) in Northern Europe.
Keeping in mind the idea of simple discoveries, devellopment of wheelbarrow in Antiquity (Roman or pre-Roman) would be a very important devellopment in not only agriculture but as well all kind of important works.
Similarily, the devellopment of an early railway in Antiquity could have long lasting impact; altough I don't expect having iron railway appearing first but on the model of the diolkos of Corinth, either in limestone or wood.
Eventually, and I'm surprised nobody mentioned it already : you could have a Roman use of
trip hammer, or if we consider they did used it, a much larger use than IOTL. We're talking important structural advance in mining and mettalurgy there.