Hi Tony,

You have a couple of options:

Option 1:

1. Enter one long task "build aquarium" and set the duration to 8 weeks.
2. Set the Task Type to Fixed Duration, non-effort driven. (use the Task Form in the lower pane). This will prevent Project from recalculating the task duration when you enter work.
3. Then using the task form, assign the 2 days of work for your friend and the one week's worth of work for you.

Project will calculate the assignment units needed to spread the work across the 8 weeks. It will calculate a pretty low percentage spread evenly across the duration. If you are tracking, you can then enter the actual hours as they occur.

Option 2:
Be a bit more detailed. Break the 8 weeks into more discrete pieces. So the first tasks where your friend is helping you for 4 hours -- set that up. Then define the "waiting tasks" as needed and don't assign anyone to them. This will allow the breaks in work and be a bit clearer about when during that 8 weeks your work is required.

Regarding the non-project projects -- I personally wouldn't. The advantage of an 80% allocation is that you are automatically saying that there is 20% in "other work" that you don't need to track or plan as closely as other projects. If you want to use 100% allocations, that is fine as well -- and you can view your "free" time by adding Remaining Availability to the Resource Usage view to see when you can slot in the other work.

I hope this helps.
Julie