Randomly generated hills?

So this is a quite strange request im working on this game and i'm pretty far however I do not like the generation system (Voxelmetric) as it uses chunks andthus I can't place blocks on the terrain.

I need to know or a push in the general direction how to get random hills to spawn in the sea ( in other words I don't need a complete generated terrain as that would kill fps I only need some basic hills made out of cubes that spawn in around 100m from eachother)

So I was thinking and thought of 2 ways to do this:

1. Have prefabs of islands spawn 100m from eachother infinitely using the character as a ''spawn generator'''

2. using a grayscale/heigtmap to have islands spawn infinitely.

Anyone knows how to do this or have any link towards a page that can help me?