Just explored yesterday! Wouldn’t exactly call it a hike, it’s a fun day out when the weather is good or bad - we had light showers when me and my girlfriend went! The lava caves are pretty cool to explore (tip head to the furthest one as that cave is tall enough for you to stand in at one point). I would say if you wanted to get your workout in, perhaps after the summit climb, head down to the Islington Bay Wharf and perhaps walk over to Motutapu Island (this would make it a day trip though).