New Zealand is the most handicapped accessible nation in the world, they put the US to shame. I know, I'm handicapped.
I took my mother there on a 2 week tour when my mom was 84.
There are wheelchair accessible bathrooms in tiny petrol stations on the southern Island.
There are non-stop flights to NZ from L.A.
I would look into a cruise on a MEDIUM-sized cruise ship like Celebrity or Seabourn, avoid the mondo ships like Norweigan or Carnival. You want a ship of about 450.
The reason I don't recommend the small lines is I'm not certain that all of them are handicapped accessible.
I think a medium sized ship w/ educated/expert lecturers would be terrific stimulation for your dad. Think about a cruise in Asia, southern Europe, the South Pacific (direct flights to Papeete Tahiti are only 8 hours nonstop), avoid Caribbean or Mexico... to avoid a lot of partyers or families.
Look into the Galapagos or if you can afford it Antarctica... National Geographic cruises go through Seaborn or one of the other upscale lines.
BTW, he won't be the only 90 y o on board.