(If you're specifically looking for how to get the good ending, scroll down!)

Seascape Paradiso has a somewhat unusual structure compared to a lot of visual novels, so this walkthrough doesn't have exact "pick these choices" lists.

Bad Ending - Sidetracked

Pick "Just follow Echo" when you're given the option.

Neutral Ending 1 - Missed Connections

Don't pick up any of the items you come across while exploring.

Neutral Ending 2 - Indefinite Farewell

Pick up at least one of the items you come across while exploring, and after reuniting with Echo, choose "Don't give Echo anything."

Good Ending - See You Soon

Pick up any of the items you can discover while exploring, and give one of them to Echo after reuniting with her. This ending has three different variations depending on which item you give her.