Any ideas for activities for my very active 96 year old grandfather recovering from a broken ankle?

What were his favorite activities before he injured his ankle? Can you come up with versions he can do with his current limitations? For example, if he loved gardening, could he sit and make up some planters with a variety of plants, for gifts? If he loves walking, can you take him for walks in his wheelchair?

If there are a lot of family photos sitting around in boxes, could he identify the people in them and the approximate dates?

Perhaps there are some chores he could take on now, such as folding small laundry items, matching up socks, cleaning out a dresser drawer that is placed on a table for him, polishing metal items ... anything that takes some energy and would really be useful.

Does he like to read?

Board games and card games, it lets them socialize and have fun with other despite their limited physical ability. There are some board games which stimulate the mind and enhance thinking. You may also feel free to check this list which I have written for seniors, disable and long-term care recipients at infolongtermcare:
http://www.infolongtermcare.org/5-fun-indoor-activities-for-ltc-recipients/

Chess. Checkers. Chinese Checkers. Gin Rummy. anything interactive with another live human. If he is an old veteran, contact your local VFW or American Legion and ask for someone to visit him.