How long will you be in each port? I'd say you could do the small islands on your own, or just get a taxi and have the driver do a quick spin. It's mostly beaches, restaurants, and casual shopping. Apia has the Robert Louis Stevenson museum but don't get your hopes up for big-city doings.
If you'll be in Suva, it's a pretty big city as Pacific islands go, so you'd need to travel a bit to get out to the more scenic areas.
Conversely, I'd recommend the excursions in Brisbane and Sidney since there's so much to see.