There are 75 Island Smashes to find, you only need 20 for this achievement. The smashes are just like the normal Paradise City smashes, except instead of being Yellow in color, they are Orange.
Here is a map of where all the smashes are located.https://imgur.com/rYJe7eS
If you collect all 75 of them, you will unlock the Toy Spirit. Its the Toy version of the Manhattan Spirit found under the Legendary Cars, aka the GhostBusters Ecto-1.