Only a couple of ideas i can come up with at the moment
Sorry to hear he is so unwell. I'm assuming these things are not for posting thru mail. Some ideas putting into interests and sections with links where possible
WALKING -
Dvd - www.walksaroundbritain.co.uk/dvd - There are 2 DVDs and between them, there are 20 walks with different places featured. That might give him places to enjoy the scenery even he can't walk them himself, it also wouldn't include any reading bar the cover which someone could help with.
May also need a DVD player if he hasn't already got one that will open up lots of other options such as comedies, old TV programmes (look up GOLD as a channel and you might see something)
I'm guessing due to past walking interests and fitness he has good muscle power in his legs which won't be affected directly by recent op, he will also be frustrated by being sofa/bed bound, I'm going to guess he is on a feeding system so is confined to one place due to that as well as the need to rest.
What about one ofwww.completecareshop.co.uk/therapeutic-aids/pedal-exercisers/pedal-exerciser-with-digital-display - can be used on the ground to pedal to maintain his leg muscles or on a flat surface so he can use it with his arms, it would help to stop muscles from atrophying and also keep his previous interest in fitness alive, This one has an inbuilt computer so with someone keeping on eye on the readings it could be competitive for him to beat previous days score by telling him, was 50 yesterday what can you do today?
GARDENING - in some cases I would suggest a terrarium or indoor garden, but as a family to club together, this is making an assumption again that he is at home in own surroundings and not a nursing home, a wheelie trolley with www.completecareshop.co.uk/kitchen-aids/kitchen-trolleys/kitchen-trolley with a couple of containers, compost (there may be small bags in the pound shop as a large one isn’t needed), seeds for flowers and small watering can. This can be wheeled to near the window once he has planted to get the light and can be a source of joy to watch as nature happens. Can also with Xmas morning be changed for a tray of holly, ivy, other Christmas flowers and then switched back to normal afterwards.
On the same vein if had the outside space how about a raised flowerbed if he was able to access the garden with the help of a wheelchair or with practical help. Flowerbed can be raised up using pallets or other options as described here. www.goodhousekeeping.com/home/gardening/g20706096/how-to-build-a-simple-raised-bed/
How about someone coming to the house to do a treatment/ therapy for him? Ear candling can be relaxing, reflexology can be therapeutic and comforting, Back massage might help work out knots where he hasn't been able to keep up his previous level of fitness.
Making a DIY coupon/voucher book as corny as it sounds. Offering to do x or y, could range from cleaning his house, taking him out for a drive in the car, watching something of interest to him even if you have no interest. This could be drawn instead of words by way of cartoons.
As he sounds like he was fit and spritely 6 months ago as walking 12+ miles regularly, he may be very frustrated at being confined and bed / sofa-bound.
Does he have a group of friends who he might like to organise a "party" with, you could host a group of his friends all in one go as he will probably get tired easily and enjoy all the chit-chat, although he can't drink or possibly eat, it would be social and all organised and cleared up someone else. It might also be a time to hear a lot of stories about his past when he is with you as its better to spend time with people while they are here and as he is terminal than meet them after he has passed. Time to make memories and have fun, far better while he can take part.
Like PP fancy pyjamas or maybe track suit or whatever he would have worn to the fitness classes of old, he is at home and they are comfortable and acceptable to wear in house and also in bed if wanted, slippers or slip on trainers as a substitute for his walking boots.
Just a few ideas at random