Well, I expect there are plenty of other resources that would qualify for this title, depending on what I am doing at the time.
However, my bag of 35 small plastic bottles like these (I got them in apple, pear, apricot and mango flavours as well)
have proved their worth over and over again. Once I had consumed the contents, washed them out, and left them to dry, I put about a tablespoon of rice in each and screwed the lids down tight.
So far, I have never had a problem with the children opening them up and spilling the rice. I expect this is because I discuss the contents, and the fact that you can see the rice through the clear base beforehand. And maybe the threat that if they do spill the rice, then the lesson becomes “pick up all the rice” instead of music.
I use them for creating soundscapes (shaking gently, swirling, shaking hard, scratching thumbnail around the caps, taping the bases or the tops together), listen and copy, zum gally gally, or just keeping the pulse in time to music.
Brill. cheap(ish – for fruit drinks they are a bit pricey!), lightweight, portable. Just the job, from Early Years through to Year 6.