I did a week on Kauai for my honeymoon in 2018. I fell more into the poors classification, although we stretched it for our honeymoon. Stayed at the Sheraton Kauai, so don't have much of a recommendation for lodging outside of a resort. It was close to Poipu Beach, which got crowded, but was pretty good for snorkeling.
Fish express near the airport is a great place for poke. As others have mentioned, a lot of the Kauai stuff can be cheap, as it's mostly nature stuff you can do on your own without significant cost. Waimea Canyon is beautiful. You can drive up past that to do some hiking along ridgelines overlooking Pihea Valley. If you're real adventurous, you can continue into the Alaka'i Swamp and try to find some native birds that will probably be extinct soon.
We didn't get to do the Na Pali Coast hike, because crazy floods hit right as we arrived and fucked anything past Hanalei, but they look amazing. I'm sure Hanalei is more stunning when not shrouded by rain clouds and devastated by flood waters. The Kilauea Point Lighthouse is a cool overlook with a large seabird colony and the famous Hawaiian Geese. The rum distillery near the airport was cool to check out as well.
All the more expensive stuff (helicopter ride, fancy dinners, all-inclusive resort, horseback ride, zipline) are things I wouldn't have included if I was planning a trip solely for myself.