We're probably all pretty new to GDK so I'm certainly not more of an expert then you. I'll share with you what my experience is so far:
Activities: I only use these when i want to create Immersions. I think the only reason to decide if you want to have multiple or one activity should be readability. If an immersion is all you need then you can start your activity directly instead of using a service.
Services: I found it useful to have one central service. Especially in projects where I have state full objects with a lifecycle that spans multiple activities. In case your navigation depends on the state of e.g. a connection it also makes sense to let the service manage the navigation e.g. start new activities. Activities can interact and provide results to the service with whatever method is appropriate e.g. Intent, Broadcast, Binder, Singleton... I usually go through the list and take the first one that fits the requirement. If i can do what I want to do with intent's then that's usually my first choice.
LiveCards: A central service would also be a good place to manage a livecard that you want to share across other activities. Again the method to expose the live card interface to activities depends on what kind of interface you want (abstract vs direct).
Let me know what you think.