You mentioned a neuropsychologist- has your husband had the testing done? If yes, I recommend you schedule a return visit of just you and have them show you your husband’s testing answers and explain the details, comparing and contrasting the results with the “norm” for his age. This was totally eye-opening for me. I knew then I had been that frog in the boiling water, time goes by and without realizing it really, you make bit by bit adjustments. The testing also creates a baseline that repeat testing in a future year will provide hard data for the changes. My husband enjoyed the testing- said it was like doing different sorts of puzzles and word games.
As far as activity, I vote with other commenters to just work with what he likes and let him be. My husband, too, sleeps a lot- I read somewhere that the brain realizes something is wrong and attempts to “right it” during sleep. He used to play hours of solitaire, but can’t figure the game now. He likes drives in the country, so we go to drive-throughs (in person restaurants can be overwhelming/stressful)- Dunkin for coffee and a doughnut or Dairy Queen for ice cream or McDonalds for a shake, and then take a drive. A drive makes for easy and good conversation about what you see, memories of what happened at that spot, talk about the landscape, what’s new with the season- here the dogwoods are blooming, farm fields are being plowed. I drive by his old workplace and he’ll launch into a funny work story (have heard them all a million times, but so what- we laugh and laugh).
I wouldn’t worry about keeping him busy, “go with the flow”. We’re here for you.