Discovery bottles are any kind of bottle (water, pumping bottles) filled with items to capture visual and auditory interest.
I used dried Israeli couscous here, but almost any dried grain could be used (rice, barley, even pasta or white beans). I dyed the couscous using food coloring. You could also use liquid watercolors.
Add a few drops at a time and stir with a spoon until coated and the color depth you want. Use a different spoon for each bowl.
The grain will dry faster if not too wet. You don't want the grain to absorb much water or it could expand and eventually mold.
Spread out dyed grains on paper towels and let dry.
Add grains to clean, dry bottle of choice. Place a small amount of glue around rim and close with lid.
Discovery bottles can be filled with whatever random objects you find that could be interesting to look at and/or make sounds when shaken. You can also stick with one theme for a bottle.
You could put small objects in and fill the rest with rice to make it a hidden objects game.