I went there for the first time last year: Oahu & Maui and we did touristy stuff and non touristy stuff but here are the things that stuck out for me.
Oahu: leave Honolulu. We stayed in a guesthouse near the rainforest on the east side of the island. Just north of Kaneohe. We rented some ATVs and drove them up the coast. If you are in Honolulu definitely do the Manoa Falls hike. If you're into history or even if you're not- go to Pearl Harbor. It's really cheap if I remember correctly but go early in the morning as the lines start around 6:30am. (It's easy to get up early in Hawaii because of the time zone difference!)
On Maui we did this awesome day trip if you're into snorkelling tours. Crystal clear waters...
www.bluewaterrafting.com is the site where I booked. Of course the Hana highway is something else to experience on Maui. As long as driving slow doesn't bother you. But there are many stops along the way including a black sand beach if I remember correctly.
Haven't been to any of the other islands but I'd love to someday.