Don't squirrel away your own archive of contacts, events, or images. Use the systemwide address book, calendar, and photo library.
Offer to add or extend existing contact and event records.
Be generous about linking to other apps for secondary tasks, and let your app accept links, too.
Too much app switching melts brains. Linking to other apps works best as an endpoint, not mid-task.
Take a "just enough" approach to web views and map views, letting people jump to Safari and Maps for more advanced features.