The best place for this would be the Hawaii forum under destinations but I'll leave that judgement up to a MOD. The Plumeria lounge is far from your gates even with the bus which runs on "island time". DL or AA lounges will be your better choice. While free the UA lounge isn't a standout. But that depends on how much time you end up having there. If you have 45 minutes to burn I'd just use those UA passes.
You may be asked to leave your room at 10:30/noon but generally that doesn't mean you have to leave the grounds of the hotel or that they won't hold your baggage until you're ready to depart for the airport. Many hotels offer a day package with fitness center/spa access. For an 18:30 flight one won't have to leave for the airport until ~16:00 depending on where you're staying in relation to the airport. That's plenty of time to visit a place and/or a nice last meal on the island.