(Graphics Mods)
-Blackrack Volumetric Clouds
-Parallax Continued
-Deferred Rendering
-Restock
-Waterfall

(Ship Parts Mods)
-All of Nertea's mods including Near, Far, Kerbal Atomics.
-Tundra Exploration
-Blue Dog Design Bureau
-Habtech2

(Planet Pack Mods)
-OPM
-Kcalbeloh System

These should basically turn your game into a new one and honestly these are barely scratching the surface at how much you can change how KSP plays. I haven't even listed the mods that add life support systems and health for your kerbals, better functionality for surface bases and colonies, more strategems for your administration building and so forth.

After putting a bunch of time into Juno, which has a visual scripting language that allows you to automate the rockets but has engine limitations that give it less of a fun factor than Kerbal, I found this mod that lets you write scripts for KSP.

https://ksp-kos.github.io/KOS/

It's a little bit awkward to get going but if you want to get more advanced and have your rockets get themselves to their target orbit etc it's pretty cool.