I'd say your time is already well packed, without more suggestions from me. However, one short stop on your way down the east coast would be the Moeraki Boulders. Catch them at low tide for the best viewing. Don't stop at the touristy beach access with the gift shop and visitor's center. The ask for money to use their stairway to the beach. Just a wee bit south of that is free public access with a parking lot. You walk a few hundred yards further, but it doesn't sound like you're adverse to the exercise.

Have fun, and good luck on your reconnaissance trip.