Here's another idea. You should check to see if the old Bali Hai Restaurant will be re-opened in time for your function. They have the room, and I consider the view across Hanalei Bay even better than that from the St. Regis. My experience with them is they really do try to meld Hawai'ian foods into their menu, not just for show. Their taro hashbrowns were a delicious example.
As of 11/10, they were not open. However, when I asked, I was told "It could be any day."
Bali Hai Restaurant
5380 Honoiki Rd, Princeville, HI 96722
Roy's Tavern at Princeville (at the Prince golf course clubhouse) is pretty roomy and casual. There's also a function room upstairs at the clubhouse which might be run by the St. Regis. But, on our two visits in September the food wasn't very good. Maybe that's changed by now. They'd only been open a couple of weeks when we were there.
On that side of Kaua`i, you might need to talk to the catering department at the St. Regis. While a wonderful area of a great Island, there has been a dearth of larger restaurants there. Maybe that has changed, but considering the fact that none of the locals has commented yet, makes me worry. In Ka`pa`a, you might want to look to Josselin's, as he has had larger rooms in previous restaurants. Have not been to the new one, but it has gotten very good to great reviews. Still, that is a bit of a drive, especially if wines and drinks are involved.
That beautiful North Shore area just seems to be bereft of really good restaurants, and it's been that way for a long time. Other than some great little local restaurants in Hana`lei, everything lasts for a season, or two only.
re: Bill Hunt
Josselin's is WAY too far from Hanalei for a rehearsal dinner. Also, I don't believe there is any private room at his tapas restaurant as it is upstairs and rather crowded. The ambiance too would not make for a good, intimate setting IMO.
St. Regis is going to cost you quite a bit, undoubtedly. Maybe Mediterranean Gourmet does events? Not sure. I think for everything else you would need to head down south a bit. Maybe try Hukilau Lanai in Kapaa?
5-7132 Kuhio Hwy, Hanalei, HI 96714
, Kapaa, HI 96746
Oh I do agree on the proximity, but out that way, I did not have anything better to offer.
As for Med. Gourmet, I am unfamiliar, and hope that it works for the OP. Seems that Bellachefa thinks so too!
Over the years, there have been so many posts on that side of Kaua`i, and so few recs. When we were last up that way, we did very casual in Hanalei, and I did not think that any of those places would fit the bill.
To the OP, good luck, and a wonderful experience,
re: Bill Hunt
Gaylord's (Kilohana Plantation) can handle parties that large, and they're known for closing the entire place just for a private party. Not especially casual by Kaua'i standards, but casual by mainland standards.
You might also TRY to see if the Lighthouse Bistro in Kilauea would do it, 40-50 might be pushing it, but if you eat buffet style, they might be able to accommodate. This is a nice, casual place, that strikes a good balance between local flavor and fine dining.
There is also Casa di Amici in Po'ipu (former` tenants of the Island Bistro site). I think they could do that many, but it would fill the place.
There is Duke's down at the Marriott on Kalapaki Beach. They could do it easily.
Finally, I would warn you off the St. Regis. No offense, but even after their namechange/remodel, the place STILL has missed the mark. It's hard to describe, but they're a hotel IN Hawai'i, just not a HAWAI'IAN hotel, if that makes sense.
Hope this helps.