Google provides a variety of \'cards\' for Google Now (http://www.google.com/landing/now/). Is it possible to create your own cards? The system looks pretty modular, but I haven
There is a work-around that will soon allow you to place cards in Google Now's stream at a particular time or a particular location: Use Google Keep (https://drive.google.com/keep/)
You can create a new card at Google Keep with a time based or location based reminder, depending on which the relevant card will show up in Google Now.
Since Google Keep is now in Drive, the API is expected to be available soon (keep a lookout for it at http://discovery-check.appspot.com/ )