First off: track as much as possible! You never know which workflows you’ll want to build in the future.
Additionally, make sure you’re tracking activities at every stage of a user’s journey (the initial sign-up, profile completion, purchases, social sharing, etc).
Once you’ve tracked as much as possible, we recommend setting a goal for each triggered email. For instance, you can send a triggered email when a user moves from active to inactive as a way of re-engaging him with your brand. Another potential trigger: new sign-ups. You can create a series of welcome emails to introduce yourself to newbies (these series are often broken up into 2 or 3 parts).