Only use Gatorade or G2 bottles for the sensory bottles they are a heavier plastic so they hold up better and keep a unison look so everything looks good. They come in different sizes you can find smaller ones for small hands if you need to.
Make sure to ALWAYS seal your bottles securely! I cannot emphasize this enough. Unless you want your bottle opened and the contents poured out. SEAL IT… with glue. Duct tape can be peeled off by a determined kiddo. Glue is much harder for them to open up.
Fill a small plastic bottle with corn syrup (I used hairgel below because I was out of corn syrup, colorful letter confetti and some marbles. Use hot glue to make sure the cap stays on. The marbles add interest and break apart the letters if they clump together.
Or use other substrate such as these cool tiny styrofoam balls I got at the Dollar store.
I still used one of each foam alphabet letter in this bottle. The kids roll and turn the bottle to find the letters.
For a list of ALL my amazing DISCOVERY BOTTLE IDEAS click the link or the image below