Most Apple Watch apps are really just a front-end for apps running on your iPhone. For example the watch uses GPS on the phone to know where it is. So, while the health app on the watch can track your heart rate and steps, without an iPhone, it couldn't track your course. Also, the watch doesn't have a cell radio to make or receive calls without an iPhone attached. Finally, you need an iPhone to set up the watch in the first place.
So, what can the watch do without your iPhone nearby?
- One feature I couldn't demo in 3D was the ability of the watch to be used for Apple Pay. Once it's configured, you don't need to be connected to an iPhone to use Apple Pay.
- You can use your watch to board airplanes (if the boarding pass has been previously downloaded from your phone.) Electronic tickets can also be used for theaters, etc. if you have previously downloaded them.
- You can listen to music, podcasts or audiobooks previously stored on your watch using a Bluetooth headset. 2GB is reserved on the watch for this kind of thing.
- If you have a Bluetooth-enabled smart lock or PC lock you should be able to use the watch to unlock it.
- You can track your workout on your watch - then sync it up with the Health app later.
- You can control your AppleTV or iTunes on your Mac or PC using the "Remote" app on the watch.
- The watch, stopwatch, timer and the alarm functions all work without being connected to the iPhone.
- You can read previously downloaded emails and text messages without the iPhone connection.
- You can view your calendar without your phone connected.
I hope this helps - if you have other questions, just ask!