In response to 140654: Kekaha Kai State Park, posted by Jay S2 on Tuesday, 28 February 2017, at 3:53 p.m.
You can find a few youtube videos of some unfortunates who have overstayed the closing time at Kekaha Kai. It involves working by headlight to essentially vandalize the gate in order to get out. Plus, the rangers will usually make the rounds in order to move people out of the park well before sunset. This assumes you use the "official" access road a few miles north of the airport. Time are strictly enforced. A bit of research will likely locate better options, and sunsets are spectacular wherever you end up.
That being said, it's not uncommon to find cars parked in the dead of night along Queen K highway closer to Kua Bay where the north road leads to Makalawena. I can't say as I'd like to walk this one at night, but apparently many find this inviting. Kua Bay itself is accessed directly across from the West Hawaii Veterans Cemetery and is open till 7PM.
You might instead consider Beach 69, accessed from Puako Road. That one, part of the larger Hapuna Beach Recreation Area, is open till 8 and affords many scenic opportunities. Really nice IMHO.
Closer to town, it's hard to beat Old Airport for sunsets. Always a safe bet.
As for astrophotography, I'd recommend going mauka towards Waimea and "the saddle". The Big Island has fairly predictable cloud patterns and among those are these: Kailua-Kona will be partly to mostly cloudy in the evening. Waimea will be better. The saddle...let's say for convenience sake Mauna Kea Recreation Area...will be stunningly dark. Even better, the Onizuka Visitor Information Center at 9200' elevation on the Mauna Kea Access Road often hosts stargazing with highly accomplished astrophotographers and amateur astronomers. It's worth checking the website: www-dot-ifa-dot-hawaii-dot-edu-slash-info-slash-vis-slash. (avoiding posting a direct link per Kweb rules). If you're looking for dark skies...some of the best on the planet...this is the place and NOT TO BE MISSED.