Another rec for Tantalus - I rented an Allez from The Bike Shop in Honolulu and did the Tantalus climb when I visited 2 years ago. Most people do the climb clockwise as the descent will have less turns. It’s not too long either, so it won’t wear you out or take you away from the rest of your vacation.
And like others have said, traffic and road conditions can be a problem, so go early in the day.
Also there’s a lookout point on the east side with nice views of the city, and if you come back in the evening it’s a nice place to catch the sunset.
