Road riding from Kaneohe, I would take Kamehameha Highway along the north shore as far a Haleiwa and back. If you go all the way to Haleiwa, stop for some shaved ice in Matsumoto's. Depending on exact start location, it is a 60-70 mile ride. Caution, you will most often have tailwind heading towards Haleiwa and headwinds back to Kaneohe, so do not overdo your distant on the first half of the ride.
Shorter nice turn around points are Waimea Bay - 12 miles shorter (ride just past the bay to the hillside for the best view looking down on the bay), or Kahuku town - 30 miles shorter but not as scenic stop.
The mini-marts are cool with you stopping for just ice water if you do not want to buy any sport drink.
Land of the Free, Because of the Brave.