Now, I call this an Aquatic Sensory Bin because the base material is aquarium gravel and almost everything else in it are things from the aquarium aisle at a pet store.
So, for your base material you can use any color aquarium gravel you'd like, I love the pretty multicolored one we used. I also added multicolored fake jewels, and a artificial piece of pink algae. Your kiddo has probably never got to experience something that feels quite like this yet, so it's sure to make for an awesome sensory experience.
Although it doesn't go with the theme that well I wanted to introduce my daughter to that wooden frog that's in the bin. It makes noise and enhances fine motor skills so I had to add it. You could add a fish bath toy, or more things from the aquarium aisle at your local pet store.
To add scooping and pouring to this bin since so many toddlers love to do that, just provide a plastic measuring spoon (tbsp size is great) and a dish or bowl. I give a purple plastic 1 tbsp measuring spoon, and a plastic yellow pineapple shaped dish that I found in the dollar section at Target.