From which activities am I (not) excluded by not registering for a conference/symposium?
The terms "conference", "workshop", "symposium" might have specific meanings in some specific contexts, but in generall, they are rather interchangeable terms.

Registration fees are typically intended to cover the cost of the conference, and maybe to make some profit for the organization running it. In any case, part of the fee will cover the costs directly attributed to the person attending, and other parts go towards general costs.

Even if one is not causing any specific costs, it is often frowned upon to attend any part of a conference without being registered, unless this has been cleared with the organizers.

In your case, this happened, and the question is what exactly you might attend. The rule of thumb is that if there is a cost-component per delegate, stay away from it: Coffee breaks, lunch, conference dinner, conference excursion would be typical examples. Also, do not take a conference bag.

For most conferences, the rest will be talks (in various incarnations), and maybe poster sessions. If you find something else on the schedule, you should ask the conference organizers directly.