Hilo is a very quiet and rather old-fashioned little town. There is considerable local beauty to be found - the 'downtown' area with the harbor and park and looming visage of Mauna Kea is charming and picturesque. If you're the type that likes fine dining and nice wine with lunch you may well be disappointed, however. We didn't take any excursions there (having visited the area numerous times previously) but I know that the other areas of the Big Island are full of amazing sights (e.g., Volcanoes National Park). The highlight was a local plate lunch, served by a guy in a food truck in a parking lot, that was fantastic!
Having lived in Honolulu for five years we chose to rent a car and spend the day visiting friends, etc. I'll just point out to first time visitors that Oahu has so much more to offer than Waikiki and Pearl Harbor (not that there's anything wrong with those places)!
Nawiliwili is a charming little harbor near Lihue. We chose not to take an excursion there either, instead taking the short walk to the nearby beach. Being adjacent to the Marriott there is a nice little beachfront restaurant and a small shopping area featuring the usual t-shirt stores, etc. Kauai is one of the most beautiful places imaginable and I'm sure the first-time visitor would love a trip to Waimea Canyon or a helicopter tour.
This 'snorkeling' trip was the highlight of the trip. I use the quotes because while the snorkeling was lovely, we had the once-in-a-lifetime experience of being "mugged" for an extended period of time by a pod of humpback whales that surrounded the boat and put on a fantastic show for us. Captain Dave and his crew, being experienced whale naturalists themselves, provided terrific information and seemed as blown away by the experience as we were. Obviously one has to be extremely lucky to have that happen but the crew were really fun and engaged and I'm sure they would provide a good time no matter what.