Mummies in the Morning by Mary Pope Osborne

The tree house whisks Jack and Annie to ancient Egypt, where they explore a pyramid and meet the ghost of a queen who needs their help. As they get lost in the dark, maze-like chambers, they must solve clues and face their fears. With the help of a magical cat, they find their way out and complete their mission. The story combines mystery, ancient history, and a touch of the supernatural. It's a spooky but exciting visit to a world long gone.