What can I have elder with mild dementia do at home instead of sleeping?

You say that she has "limited interest" in magazines. I would greatly encourage her to enjoy magazines with a specific content. For example, if she showed interest in interior design when she was younger, then by all means get Elle Decor, Architectural Digest and perhaps Traditional Home. My favorites are This Old House and the New Yankee Workshop. Sit with her and turn the pages.....just suggestion that I've seen work with some seniors.

My mother (92, mild to moderate dementia) spent the weekend with me. She was confused by the change in surroundings and anxious the whole time. She mostly watched game shows and did her crossword puzzles and tried to keep me in her sight at all times. At one point I asked her if she would fold my panties for me to put away. She perked up and was eager to do it. She will be spending one weekend a month with me and I know that I am going to save that particular load of laundry for her to fold! Maybe I'll get her to sort socks, too. I think she likes that they are obviously tasks that help me.

She's the Sweet Queen and when I told her I was making a date cake she offered to cut up the dates. I had bought them already chopped, but for her next visit I will try to have something on the menu with a task she can do.

Dementia which is a serious brain condition could have been controlled or prevented if only we are aware of the things that could hamper it from occurring at a very early time. Engaging her to simple yet enjoyable chores like cooking, gardening, and things that would interest her could really help.