The Sui should keep the Grand Canal, probably building it same as historically, because that was a good idea and China needs a transport path to connect the Yangzi River Valley to its Guanzhong Heartland. It could probably have kept the wars against Koguryŏ, because the terrible Sui military strategy could have been changed to a better one, though I don't know if the Sui would have realized these issues and they certainly weren't helpful. Emperor Yang probably should have refrained from building that network of palaces and the capital at Jiangdu.
A lot of the Sui's failures were failures of situation (flooding and famine) or personality (Emperor Yang). Even those could be overcome: if one takes out Emperor Yang leaving Luoyang maybe the Sui could survive if it pits the rebels against each other. But it would be a lot simpler to just avoid Emperor Yang. Perhaps an alternate succession, with one of Emperor Wen's other sons taking the throne, would be enough for the Sui to survive longer.