I noticed this was posted back in April, it's now Nov. 30th, but hopefully you are still checking in. First, I wanted to say that I was blessed with an 'Oma' too You are very thoughtful and kind to be thinking of creative outlets that she would enjoy. She's lucky to have you.
Have you thought about 'finger knitting'? Full Disclosure: I just heard of this and don't know anything about it other than it was suggested as an activity for myself (I have an illness/disability that confines me much like your Oma) and my 6 year old son that we could do together snuggled in bed This friend is a crafty person, and knows of my limited mobility, cognitive difficulties (e.g. hard to follow patterns now ;( ), and lack of fine motor skills. She also knows how I treasure my time with my son-shine, and thought it might be a fun project. Other than that, I have not yet researched the how-to's on 'finger knitting', but I'm hopeful it will be something that we can both do together with minimal frustration. Best wishes to you and your Oma!