A good question.
A local answer.

Compare the actual tour duration, departure point and return time. If the island option gives you little time ashore, save it for an overnight visit.

Platypus require particular habitat and patience; do not build a tight day around seeing one. A guided nature outing may help you understand where to look without disturbing wildlife.

If dolphin kayaking is the priority, book with a licensed operator and check the launch point, tide and weather policy. Keep a different coastal plan if conditions are unsuitable.

Build in shade, seating and a late lunch, especially in January. A short guided food or history visit can add context without a demanding trail.

If you want wineries or hilltop viewpoints beyond the town, compare their last-mile transport separately. A bus to Hahndorf does not make every Hills attraction walkable.

Check path grades, shade and the final return service. Leave one day in Glenelg for weather and a relaxed waterfront walk.

If you need step-free access or parking, check the latest official visitor directions before setting out. A nearby construction notice may concern the arcade rather than the market hall.

Check the short-range forecast for each stop just before departure. Adelaide, the Murray and Tasmania can feel different on the same itinerary.

Choose one evening performance or tasting and make a reservation. Keep the other afternoon flexible if the organised tour runs late.

Use one full day for Barossa if wine and food are a priority. Decide between a designated driver and a tour before choosing stops.

Check the train-to-wharf walk and return timetable before booking. If a cruise is the priority, ask the operator about duration, boarding access and cancellation terms.

Keep one evening unbooked for a show you discover after arrival, but reserve popular performances in advance. Glenelg transport back after the curtain matters too.