Any ideas for something fun for rising 3rd graders to reinforce correct punctuation/capitalization/spelling and other grammar rules? Thanks.

This wouldn't help with conventions, but for parts of speech you could try sneaking in some grammar while you are in the car. Here is what my kids are listening to right now:

http://teachingmybabytoread.blog.co...ulum/first-language-lessons-for-the-wtm/

A book I just bought told me kids learn through correcting other people's papers. I guess you could write your own short stories and put errors in them for the young'uns to correct you on.

parts of speech - Mad Libs
various grammar - School House Rock

one of my favorites:
(Interjections)

I think the advice to write short stories for the kids to correct is great.

When my son was little, I would write short stories featuring him, his friends, and his favorite characters from books and movies for him to correct. It combined two of his favorite activities: hearing stories where he was the hero, and proving his mother wrong!