I haven't been to the Big Island, but I've been researching it extensively for a trip there once traveling is safer.
Something to know about Volcanoes National Park on Hawaii is that after a lot of activity back in 2018 there is no more visible lava. In fact, a lot of the park is now off limits because of damage from the eruption. From what I can tell though, it's definitely worth visiting and there is still a ton to do! You can look into the lava craters, see a lava tube, see some steam vents, and lots of other cool stuff. Definitely a full day activity, and more if you are really into hiking. While it is disappointing that there currently isn't any lava, it's nature, you just got to roll with it. There is no predicting when lava will reemerge and when the volcano will become more active.
As for stargazing, I found
Stargaze Hawaii that does stargazing at the beach level. They are currently doing it a few nights a week in the parking lot at the Westin Hapuna Beach. I'm sure there are a few other companies that do a similar thing. From what I've seen on their website and from reviews, you can see a lot from the beach level. As others said, you can probably do something similar at Aulani or throughout Oahu, but I've heard the Big Island is one of the best places to go stargazing, so if you make the trip out try to take a look.
Personally, I'm planning to do a split stay on the Big Island. I want to stay in a cheaper hotel the first half of the trip and do the more "active" things like Volcano National Park, the waterfalls, the botanical garden, etc. Then, I'm going to a resort for the beach and relax portion. If I'm going to be out exploring most of the day, I don't want to pay a lot of money at a resort I wouldn't be spending much time at. I also want to do the adventurous stuff first so I can really relax after. So if you are looking at doing the Big Island, maybe spend some time there and check out the other things to do on the island then got to Oahu for Aulani and do the beach and pool part of the trip.