Grand fetch quest

I guess we were double crossed

Once you get the crown you have to get out of the swamp cave the same way you came. The next step is to return the crown to the king in the ruined castle. As it turns out he is actually Astos in disguise so you have to fight him. Astos isn't that strong of a boss but he can throw instant death attacks at you so you want to beat him as quickly as possible. Once you defeat him you will get the crystal eye.

The ultimate energy drink

If you remember the witch Matoya was missing her crystal eye, just like the one you got from defeating Astos. So next you need to take your ship up to her cave and return the crystal eye. If you were wondering why there was a stone harbor right next to her cave, it was for your convenience. Give her the crystal eye she will be so grateful that she will give you the jolt tonic (herb in the Nes version).

So was he keeping it on him while he slept?

You take the medicine that Matoya gave you to the sleeping elf prince. He will wake up right away (and in the android version get out of bed) and in according to the ancient prophecy give you the mystic key.