Here are some sites for you to explore:
For all elementary grades, but specifically for those in the primary grades:
For the intermediate grades:
Online Sequencer ~ create your own grooves using different sounds
Online Piano Keyboard ~ a virtual piano
These next links, while free, will require you to download an app or create an account:
SmartMusic ~ this is a great tool for band & orchestra students, vocalists, and composers.
For the students who would like try writing songs using music notation, here are three free music composition tools (which you will either need to download or create an account):