One thing that I never figured out while playing the demo

[Christi]

How do you open those jelly fish things?  At the start of it the game said to match the sing the symbol, but when I opened the song menu I could never find, what looked like to me, the sun with a dot in it.

I just bought the full version and stopped playing at the start when I sat here for around 30 minutes trying to figure this out.

[Zaratus]

You mean the jellyfish creatures that float around?  Pretty sure all they are for is killing them to get a glowy health restorative, and occasionally they drop Jelly Oil for cooking.

Or there are the colored plants, that you sing the same color of, to open up for items?

Can't think of anything else that you might be referring to.