I presume your wife knows her physical limits and won't undertake anything physically too strenuous. Other than that... the only places I would say not to go are the high mountains (above the Visitor Information Station on Mauna Kea, or above the end of the roads on Mauna Loa), or, as you've already noted, up close and personal to volcanic gases.
Most of Hawaii Volcanoes National Park should be just fine; even the Jaggar Museum, which overlooks Halemaumau Crater and its steam plume, doesn't usually get anything nasty unless the prevailing winds die down.
Also fine would be waterfalls (Akaka, Rainbow, etc), botanical gardens, museums, beaches... it all depends on what you're into.