I actually had basically this exact situation. I played a lot online, and got ahead in Dominion; had to help my regular IRL game partner catch up with me.
Which sets do you have? At the time, I had Base, Intrigue, and Seaside. I went with the following sets:
Minion Demo:
Cellar, Feast, Festival, Throne Room, Pawn, Minion, Upgrade, Lookout, Warehouse, Caravan
Library Demo:
Cellar, Festival, Library, Remodel, Ironworks, Pawn, Fishing Village, Haven, Lighthouse, Warehouse
Laboratory Demo:
Cellar, Feast, Laboratory, Spy, Witch, Pawn, Ambassador, Haven, Caravan, Warehouse
More complicated multi-part engine:
Chapel, Cellar, Militia, Thief, Council Room, Throne Room, Market, Festival, Workshop, Fishing Village
...actually, I may as well just share the googledoc I made at the time.
https://docs.google.com/document/d/1TRVImznfSm9ZqIaXK0zq0T6LkhxMmcnjnxCmGa2HysI/edit?hl=en_US . Obviously, if you have different sets or are playing on Isotropic with everything, you'll want to use different ones, but the stuff I had there (including the explanations) worked pretty well for me for getting my partner up to speed.