Halloween // Costume ideas for a senior woman who doesn't usually dress up?

My brother and my future SIL are having a halloween party wedding and costumes are strongly encouraged. I'm all for it and my kids are stoked! Wedding theme is mystical/magical vibes, fae/dragon/elves/fae wilds, etc, but costumes don't necessarily *have* to be "on theme".

My grandma, an 84 year old woman, doesn't dress up but doesn't want to feel like the odd one out. She's the kind of person who doesn't even wear jeans out of the house, she wears dressy pants and blouses or dresses. Can't restrict mobility since she's recently started needing a walker occasionally.

Ideas?