Dan is right about Hawaii Volcanoes National Park. In fact, there are a number of very spectacular hikes which are free of vog and quite doable for a lady who is expecting. The Rangers in the visitors center near the park entrance are exceptionally helpful and friendly, and I know they can make some suggestions that would work for her and still give an amazing experience. FWIW, the Park has closed the areas that are prone to dangerous levels of fumes, and that is a very small percentage of the total area. And finally, congratulations on the addition to your family!