How many side trips fit into eight days based in Sydney?

I would keep four or five days for Sydney itself, then make one or two side trips. Use one city day for the harbour, one for a ferry and beach, and at least one for neighbourhoods and meals without a fixed timetable. The Blue Mountains are a straightforward rail-based day trip. For a second change of scene, choose either the Southern Highlands or the Central Coast according to your interests and transport; both reward a slower pace more than a rapid checklist.

Keep one flexible day for weather or a place you discover after arriving. If your second trip needs a car or tour, book that first and arrange the easier city days around it. Three distant day trips back to back can consume more of the holiday than the destinations themselves.