What about a "How To" demonstration?
How to make a sandwich; How to plant a flower; etc.
I like all the suggestions but I particularly like the “how-to” idea.
I’ll also add poetry unit: limericks, haiku, biography, couplet, etc. I used a simple unit from TPT geared for 1st graders because it had writing templates. I simply raised the standard regarding Vocab, punctuation, etc to make it 3rd grade material. It was a simple fun engaging way to introduce different types of poetry.
I also taught 3rd and I started with the opening or leading of our morning meeting to get my students comfortable with sharing. We’d meet on the carpet and then a student would lead the pledge, class rules, news, map, question of the day, etc. I also gave opportunities to share their journal writing or simply a sentence or two (up to a paragraph). I found these smaller opportunities helped a lot for when the time came for the “bigger” presentations.