Oh.
Well, one of the ingredients is a male subject's strength, and I think that is pretty clearly spelled out in the Samson and Delilah retelling in the bookstore. Same with the female subject's voice, though slightly less obvious and in the Persephone legend and the Princess and the Pea story.
The reason I thought to use the shell was because of Disney's The Little Mermaid more than anything, though, so I guess if someone hasn't seen that movie that might be a little strange, but then again people hold conch shells to their ears all the time to hear "the sea" inside, as if the sound was trapped in there.
As for enchanting the objects, I noticed the Zodiac symbols in the druid's hut (and that the Archdruid was gone), and I acted on a hunch when looking for connections between the Zodiac and the four elements on Google.
Once you get to the emerald tree and the narrator points out the moon-shaped emerald, one might easily assume that it has something to do with the silver moon ingredient, and when the candle melted into a lump of wax I almost immediately put two and two together.
The salt gave me a little trouble, but that was because for some reason I thought I had some kind of special water in the cup already and I didn't want to pour it out. But once I did, it didn't take me long before I realized how to boil it away without hurting the cup (too badly).
So there were no really blatant clues for the last two, some good, old-fashioned brainstorming was needed there, but I see that as a strength rather than a weakness. And all of the solutions make sense, so it's not like you have to try really weird things in the hopes that they will work.