The Music Zone is an awesome book in general, but it has a great song to help remember Forte and Piano. Find it here for $30.
Music K-8 Volume 16 Number 4 has two simple songs for Crescendo and Decrescendo
The Bulletin Board Lady on TpT made some cute Dynamic Candy Corn Puzzles. Students in grades 2-5 loved putting these together! The download also comes with a little worksheet for students to copy down the symbol, Italian word and English definition. Easy peasy reinforcement after doing the puzzles!
Tweet Resources made a cute Spring time bundle of worksheets. Even though it is not spring, I loved the Color by Dynamics sheet and used it anyway!
My favorite part of the dynamics lesson? Bjork! Students in all grades loved moving to her song "It's Oh So Quiet" to show the change in dynamics. I wish I had a computer in my room so that I could have showed them the video as well, but they had a lot of fun just listening to the song.