I really believe in first getting the broad brushstrokes right, so I started over with the terrain several times, changing it from a bay to an island, to a canyon, to a small island inside of a canyon. For this, I used the GAIA terrain tool, which works with the standard Unity terrain. It comes with a great selection of stamps that you can combine, and it paints the terrain textures based on height and/or angle of your terrain. After that, you can go in and paint and sculpt any adjustments by hand.

There are also a lot of procedural scattering options, but I tend to paint the foliage on the terrain by hand. At this point, there are already some preliminary cameras floating throughout the scene, and I find it useful to take them into account while painting trees for example.