Your only landbound access to the Cook Monument is this trail, which offers nice lookouts on the way down and leads right to the snorkeling cove. The descending path is an easy (if buggy) hour, but after snorkeling, the uphill return seems twice as steep (in reality it’s a 1300ft elevation gain in 1.8 miles); allow two hours to return.
To get to the trailhead, turn makai (seaward) off the Mamalahoa Hwy onto Napoʻopoʻo Rd; within the first 10th of a mile, park along the narrow road shoulder, wherever it's safe and legally signposted to do so. To find the trailhead, count five telephone poles from the start of the road – it's makai across from three tall palm trees.
The route is fairly easy to follow; when in doubt, stay to the left. For the uphill return, you should stay alert for the trail's right-hand turn back up onto the lava ledge, or you'll end up on a 4WD road heading north along the coast – for miles.