I assume your MIL lives close to you
Suggestions;
"Doesnt like mess and children in the house"
If there is a garden patch available at either house you could grow easy veg and flowers with the children
or an allotment
or a volunteer opportunity that involves gardening
or volunteer at an animal rescue centre
Join knit and natter group (you dont have to knit) locally and online
Ask MIL if she can collect or drop off parcels at the post office (this is asking someone to do a good deed, gives purpose and you may think of other similar deeds, collect brochures for holiday ideas etc)
Can MIL teach your children a skill eg cooking, sewing, music, language, swimming once a week
Age UK befriending service or Silver Line where you become a friend over the phone weekly
Join a gym
Join evening or daytime college class
Go on an activity holiday
Plan a holiday
What about taking children out for geocaching
My best suggestion is that MIL starts doing something new, because the longer she leaves it, the harder it will be to try. There is alot to be said for "you dont know what you are missing until you have tried something" and if you dont like it, move onto something else
The hardest thing to bear, will be if you receive a "NO" to everything that you suggest and this will in the long term be detrimental to everyones health and well being