Wow, this is a loaded question.
You can find the most amazing coastlines on all the islands, and without some luck of specialized knowledge, you might just drive by one without even knowing what lies below.
I don't have the time to write an epic post, but since most tourists come to Oahu first, let me just mention some wonderful coastlines on that island:
Kaena Point (for safety's sake, approach from the North Shore)
http://dlnr.hawaii.gov/ecosystems/nars/oahu/kaena/
https://www.hawaiilife.com/blog/hike...oss-sanctuary/
East Oahu
For example:
https://www.hawaii.com/oahu/attracti...le-beach-cove/
All along the Eastern coast, there are a number of equally-or-more stunning areas.
Now for Volcanoes:
While my personal favorite is Loihi, most people find it too difficult to view:
https://www.soest.hawaii.edu/GG/HCV/loihi.html
However, the HVNP is a snap:
https://www.nps.gov/havo/index.htm
Enjoy!