My father-in-law is depressed and has no daily activities to occupy him.

I have two suggestions. The first would be to make certain your father-in-law's physicians knows the severity of his symptoms. It's possible there is a medication or other treatment that may provide him some relief. There's another possibility that it's not his depression that's causing him to sleep for extended periods of time. Secondly, since your father-in-law has suffered with depression for so long, it's possible that the thought of getting out and doing activities overwhelms him. If what you're suggesting doesn't appeal to him, the likelihood of him agreeing to participate is not good. Perhaps you could start smaller. Does he respond to animals? Maybe a sense of duty to care for an animal would help him engage with something/someone more rather than sleeping so much. Maybe he would respond to a new hobby? Does he sleep as much when visitors come to his home? Perhaps a routine visit with a friend/family member/neighbor would help him engage more.

In addition to what Missy said above, I'd also suggest that in the face of feeling overwhelmed, set a time frame for it. "Let's walk just for ten minutes, and then we can be done." In this way there's no open-ended feeling of "It's too much to commit to" and instead there's a definitive stopping point. In my experience, he'll come to find that ten minutes felt good and that fifteen might be great next time around.

A friend of mine has a similar problem with her dad. The siblings take turns calling him or visiting him with specific requests or questions for him... Expressing a need for him to help them in some way. Even pretty inane things like, Dad, can you tell me what the mileage is on the car.