We started with a simple bin of aquarium gravel and a LOT of scoops and containers -- including plastic, metal, glass and ceramic. This kept Ellie busy for about two weeks with nothing else in the bin. She loved filling the two glass jars -- we even talked about transparent, translucent and opaque, and she's used the words on other things since then! She also liked making a "shaker" with the lidded box, and of course just scooping, dumping and transferring.
Later, I added more supplies for a second phase:
This included fish (Oriental Trading Company), "sea urchin" spiky balls (don't remember where I got these), shells (from our wedding six years ago!!), and sea glass beads (Hobby Lobby).
She loved the additions and switched to a lot of role play with the fish. She also liked matching the colors of the balls with the fish.