We did a similar itinerary. When we arrived at Honolulu airport, we just got our luggage and made our way to the taxi stand at the airport to hop into a cab to our Waikiki hotel. When we left our Waikiki hotel to go back to the airport, we just had the bellman call up a cab from the hotel's taxi stand. A taxi is probably not the cheapest option, but it was fast and available on demand which is what we were looking for. I don't recall exactly how much it was, but I don't remember the price being exorbitant. $50 each way, I'm guessing?
For Maui, we also stayed at the Westin Kaanapali. Because its a 45+ minute drive, we arranged SuperShuttle. The cost was reasonable, but it was shared transportation with other resorts in the Kaanapali area. I believe we stopped at the Hyatt to drop others off first, so it was about an hour's drive. On the way back, we had to call the day prior to schedule our pick-up time. They were right on time and had already picked up other passengers before us. We got back to the airport in plenty of time. The only negative about SuperShuttle is that we did have one minor issue on the way in - they had us down as arriving at the airport for an hour later than we actually arrived. Because of that, they had no room for us and we had to wait about 25 minutes for the next shuttle. I'm sure this was a fluke, but next time I'd verify everything before.
A lot of people might say to rent a car on Maui. It was a pretty easy drive, so that's not a bad idea. We didn't have a car while there and utilized the free shuttle to Lahaina from the Westin. Its also walking distance to Whaler's Village next door. We were just there for 3 nights so we managed fine without a car, but for any longer, I'd either want a car or would have had to book organized tours with transportation included.
Enjoy the Westin! Out of the 4 resorts we stayed at in Hawaii, all of which are comparable, I'd say the Westin was tied with Aulani as our favorite. We enjoyed the pool at the Westin the most.