A functional glider would do nicely I think. (Doh, they had those with Daedaleus and Icarus).

I was going to suggest a boat or a hot air balloon, but Archimedes figured that one out.

Another thought would be a simple steam engine. The Greeks had iron and knew how to work it. All the rest is knowledge that wouldn't surprise them, but they'd not put it all together.

The Greek's theory of chemistry was all kinds of messed up. There are likely lots of examples there that could be used as well.