There's a lot of non-beach hiking, gardens, (take the jeep ride to Waipiu valley and drive down to see Pele at the volcano....)
Now with that out of my system...
I was there in May. I took my mom for her 70th birthday. We went to Maui and Kauai and then spent one night in Honolulu to catch a flight out.
Maui is mostly beach and hiking. It is really touristy. There are some beautiful things to see though. One of the things we didn't do is the drive to Hana which is supposed to be spectacular. Waterfalls coming right up to the road, incredible ocean views. It is like 40 miles long to drive it but it takes like 3hours to do it. Has like 600 curves and switchbacks along it. Very hard driving and something you definetly don't want to do at night. But we only spent 2 nights on Maui and didn't want to take the time to do it. There is an incredible place to eat right at the beginning of this road called Mama's Fish House. Very expensive but probably the best meal we ate the entire trip. Mom and I ended up getting 1 appetizer, 1 entree and 1 desert and splitting them. Still cost 60 bucks. And they are used to people splitting dinners and they served us two plates with half portions on them and it was more then enough food for the both of us. We actually did that alot on the trip. That way we could try different things.
Kauai was alot less touristy and a little less expensive. But it also has alot of hiking on it. There is a canyon there that is unbelievable. They call it the grand canyon of hawaii. It is well worth the trip. There is a mountain on Kauai that is the wettest spot on the earth. Has an average of 400-500 inches of rain a year. There are spectacular waterfalls everywhere. Not as many beaches and far less commercial then Maui.
We had a great time but spent a bunch of money. We picnicked alot for lunch and most of the places we stayed had a breakfast included. So dinner was the most expensive. Mom couldn't hike much so mostly we drove and sat and looked at stuff. She would sketch and I would hike around alittle. It was really cool while we were there so we weren't able to do much beach stuff. But the snorkeling and sailing are supposed to be wonderful.
The Seven Sacred Pools, which was a tourist myth, is on Maui, right outside Hana. I also highly recommend finding this spot, but not for the pools. The trail goes way beyond the pools through a bamboo forest with the bamboo growing 100 ft tall and ends at a beautiful large waterfall. Truly a spectacular hike. Also we took our rental car on the road around the backside from Hana, which was amazing. Just like the paved, regularly-travelled road, it has a hundred and one turns, but if you take it slow, still quite enjoyable. The regular road to Hana is very lush, but the backside is dry and traverses the lava-rock area. And you end up on the backside of Mt. Haleakela. The road passes by a small winery where you can taste pineapple wine. Kauai boasts some wonderful scenery as well. I recommend the Awaawapuuhi trail out to the Napali coast, if you're up for a hike to the edge of the world. Or a drive to end of the road if you drive counter-clockwise, which I believe is Hannalei Bay. Check out the taro fields on the way. Also, as Jean mentioned, Waimea Canyon is spectacular. I'm sure you'll have a lovely time, wherever you end up.