Although it's debated who actually dubbed it the “Grand Canyon of the Pacific,” Waimea Canyon is indeed grand and not to be missed.
It's a 10 mile long, 1 mile wide, and ~3,500-foot deep canyon created over the years by ancient volcanoes collapsing and erosion from the rains that fall on Mount Waiʻaleʻale (one of the wettest places on earth).
Together you will find awe in visiting the Canyon and lots of opportunities for photos, hiking, and stargazing.
Waimea Canyon State Park is located north out of the city of Waimea about 30 mins (and just over an hour from Lihu'e) (google map) on the Western side of Kaua'i.
There are two main overlooks along Hwy 550 where you can grab some breathtaking views. And plenty of small pull-outs that offer trails and other vantage points.
(Like all things involving large drop-offs, seriously, be safe. Stay away from edges, pay attention to your foot placement, etc. Common sense stuff but I just want to reiterate it).
If you keep driving past the Canyon, you'll find yourself at the Kalalau Valley; one of my favorite spots in all of Hawai'i.
// When to Go
Any time of year is a good time to view the Canyon. Seeing it at sunrise or sunset can be grand IF the clouds play along (the canyon won't get direct sun at either of those times of day). Although, just being on Hwy 550 gives you good views east and west to see either sunrise or sunset.
The rainy season (winter) will bring A LOT of waterfalls that will create fun cascading streams along the edges of the canyons walls. And if you're up for a moderate 4-mile hike, you can make your way to Waipoo Falls and see it up close.
// A ROMANTIC ITINERARY
I suggest, if you can, staying a night or two on this side of the island, so you're able to head up into the Canyon a few times (including a trip up to Kalalau Valley). And then you can also check out Polihale Beach's 7-mile stretch of sand and sun.
You can get a room in Waimea or Kekaha. There aren't as many hotel options out here, using AirBnB or the like might provide some useful options. The Plantation Cottages is a cute choice. They do have karaoke at the bar on certain nights, even if you're not staying there. That's some of the only “nightlife” in that area.
It's a very small-town authentic Hawai'i feel on the west side of the island: you'll see people out playing baseball in the evening, you'll eat from food trucks, and you'll be surrounded by all the wild chickens that roam Kaua'i.
Back to the Canyon: Being up there for sunset and then watching the stars come out over the Canyon is unreal and magical. Take a small picnic dinner up and stay late into the evening.
Note: Because of the altitude, it can get cold at night! Bring extra layers, especially in winter. I've been there in hats and gloves (so probably below 50 degrees F).
// AN ADVENTUROUS ITINERARY
Not too much different from the romantic one above except you can add some hikes. If you want to keep your feet on the ground, that is…
One of the best and most adventurous ways to see the Canyon is actually from the air in a doors-off helicopter tour!
There are multiple reputable companies to choose from; I prefer Jack Harter Helicopters (not a paid plug or anything, I just think they do a fantastic job).
It's free to get to the Canyon (no park entrance fee).