If your looking for an idea to reinforce letter sounds, a fun way can be through books and art, especially for the younger child. The book set by Jane Belk called, My First Steps to Reading is a great resource.
I appreciate her use of the letter sound highlighted in each book as well as her use of lower case letters. If you decide to use them, keep in mind that they are fictional books, and a reminder of this will need to be brought out to the children in a Montessori classroom. After reading a book I would gather the sandpaper letter, object box, and sound book to discuss with the children. An art activity would follow with something from the story that started with that letter sound. I am including some picture ideas below to give you an idea of how I combined the books with art.