Hawaii is a great place for day hikes. Here are a few short hikes on Honolulu.
On the north shore of the island, follow the coastal road to Pupukea. This short hike up the hill was very muddy when we went (close to New Year’s Day). At the top are a few old WW2 era pillboxes and great views into the Pacific.
Diamond Head is essentially a hike up a volcano with views over Honolulu. It’s a heavily trafficked route, with folks coming in literally by the bus load.
Waimea Valley has some beautiful botanical gardens and a waterfall with a lake you can swim in (it’s cold though).
Information on the gardens and the history of the area can be found here.