Consider having them write poetry. Any form, any length. Having to choose words that pack a punch, describe a scene, are sparked by a memory keeps the brain sharp.

Have them brainstorm and jot down moments - they don't have to start with full sentences or paragraphs.

Read up on the Snowflake Method developed by Randy Ingermansson. https://www.advancedfictionwriting.com/articles/snowflake-method/

It can work for any story, really - doesn't have to be a novel.