Sea of Stars

There are 187 treasure chests in the game to collect. These include the traditional-looking chests (which change depending on the area you're in, basic brown chests in the Home World and more futuristic ones in Serai's world), the green Rainbow Conch chests, as well as the picnic baskets which have food items or ingredients inside. You will be able to use the Falcon-Eyed Parrot relic on the world map to track if you're missing anything from a specific island. Once you click on a location, it will tell you the type of collectible and which specific place it is. From there, consult the collectible guide in the Road Map to run through the location and find which one you're missing. Nothing is missable or really even hidden all that well, so it's unlikely you'll pass up more than a small handful as you play, though some require backtracking due to story-related items you'll need to access the areas they are hidden in.

Note: There are three chests you need to dig up in the Cerulean Expanse which can easily be overlooked. Keep following the beeping on every screen until it stops completely before moving on.