We're going to be on Kauai and Maui in August and was wondering if any of the higher end hotels had a day pass to purchase - just to go in and use their facilities, pool, etc. We were able to do this at the El Presidente in Cozumel - for $45 got the use of all the grounds, pool and their beach area, and it also included a $15 food/drink credit.

Love a swim up bar, and we're going to be staying in condos with just regular pools so thought I would ask here if any of the hotels have a similar deal before I try calling some separately. thanks!

Never heard of this in Hawaii but certainly doesn't mean it doesn't exist. I think you need to tell us what areas in Kauai and Maui you will be staying in. On Maui the Grand Wailea would be a place to hang out since they have amazing pools and a great beach.
Of course, Maui has wonderful beaches so pack a cooler from Kmart and head to Makena or Spreckels.

All of the beaches in Hawaii are public, even the high end hotels have access for the public to use their beach.

As far as Day passes to pools and facilities. I know that on Kauai, the Grand Hyatt Kauai has a day pass for $50. They also offer a weekly pass for $225. This allows access to all facilities including their spa facilities. Check it out at www.anaraspa.com

The day rate to use all of the pools at the Hyatt (not just the Anara Spa pool) from 9 a.m. to 6 p.m. is half the price of the lowest priced room available that day. Pretty pricey but fun and you can use the room.