Flexible content within a single node?

I'm trying to accomplish something specific with a node, but so far haven't been able to frame the question in a way that gets meaningful search results. Hopefully someone can point me in the right direction. Here goes...

I'd like to create a content type that allows the user to add activities to a single node. These activities wouldn't necessarily conform to fields, but would act more like nodes in and of themselves, with no limit to the number or type of activities that could be added to a single node.

For example, say you have a node that describes a user's workday. Each day would be its own node, and within each node would be a number of possible activities that the user undertook during that day. The user would choose activities from a dropdown, and the fields available to each activity would vary based on what kind of activity it was. So, one activity might be "Begin Spreadsheet Entry" and have fields for time and the spreadsheet name, and a notes field and so on. Another activity might be "Lunch Break" with fields for Time Left and Time Returned. At the end of the day, a user might have 10 activities, or 20 or even just 5, with the order and number of activities varying from day to day.

Ideally, some activity entries would be dependent on other activities. So, an entry for "Complete Spreadsheet Entry" could not be chosen before "Begin Spreadsheet Entry". I'd also like to be able to calculate durations between related activities, such as a duration of time spent on the spreadsheet from the example above.

Hopefully, that makes sense. If anyone has any ideas about how I might accomplish this, or at least better frame the question so I can search for a solution, please let me know.