Here are some suggestions involving the spending of little or no money.
In Oahu, you can go to Waikaki beach for no money at all and go shopping across the street at the vendors in the International Market Place for inexpensive souvenirs. You can take a bus to Huanama Bay for about a dollar each way. You can even circle the island on a bus and if you don't get off the cost will only be a dollar.
In Maui, look into free shuttle buses provided by Hilo Hatie's in Lahaina. You can take the bus to Lahaina, spend time checking out the town's shops, galleries , historic sites etc. Do some shopping at Hilo Hatties and go to one of the public beaches at either end of town and take a later shuttle back to the ship from Hilo Hatties Hilo Hatties is a large Hawaiin clothing and souvenir shop, there is one in each port. If the ship's dock in
Lihuii instead of tendering in to Lahaina they run shuttle buses to get people there
(it is about 25 miles).
In Kona, there is a great public beach just a few miles down the coast from the town.
Not a lot of beaches as this coast is very volcanic. It should be an inexpensive cab ride to the beach. There is good swimming and snorkelling there. I do not remember the beaches name, but just about anyone can tell you how to get to it.
Hope some of these suggestions are helpful as well.