Kauai in September

We’re planning on going to Kauai for our honeymoon next September. I was wondering if anyone had any recommendations for things we must do/places we must visit. We like to hike more than relax on the beach. Any must-do hikes? I keep seeing boat tours, but I get sea sick very easily. Are there any that are better than others for this? Does anyone know of the most affordable lodging experience? Anything else we should know before hand? We have been to Oahu, Maui, and the big island but Kauai looks so beautiful and we’ve heard amazing things. Our favorite island is Maui. Thank you so much in advance.