You can have your event, flight, hotel and restaurant reservations, orders and parcel deliveries, and prompt for reviews or custom actions in Google Now by adding meta-data to your notification emails:
From the other direction, you can have Google Now call back into your Android app, triggered by hot words, as demonstrated in the new version of Todoist:
https://developers.google.com/gmail/actions/google-now
From the other direction, you can have Google Now call back into your Android app, triggered by hot words, as demonstrated in the new version of Todoist:
https://todoist.com/blog/2013/11/todoist-for-android-adds-su...
I can't find any published API for the latter.