For weather and/or lighting: Climates of Tamriel
or RCRN
or Pure Weather
For Lighting: Enhanced Lights and FX
or Realistic Lighting Overhaul
Water: Realistic Water Two
or Pure Waters
or WATER - Water and Terrain Enhancement Redux
For better meshes/textures on smaller objects: Ruins Clutter Improved, Static Mesh Improvement Mod, SkyFalls + Skymills, Better Dynamic Snow, Detailed Rugs, Lorkhans Vision - Night Sky, Enhanced Distant Terrain
Better Female Models: Caliente's Beautiful Bodies Edition
or DIMONIZED UNP
Female Textures: Mature Skin
or SG Female Textures Renewal. Make sure to get the one that matches your body, either CBBE or UNP.
Better Male Models: Better Males, High Res Face Maps for Men
Trees and Foliage: Skyrim Flora Overhaul, Lush Trees and Grass
Bigger texture overhauls (most likely to impact FPS): Skyrim 2K Textures, Vivid Landscapes All In One
The last step is an ENB. There are many, and which is best is more subjective, this will also cause the biggest performance hit with a possible 20-30fps drop. If that's too much, you can try I Can't Believe It's Not an ENB.
Naturally there's much more in every category, but this will get you started pushing your system to see what it can handle with acceptable performance. You'll find each by googling their name followed by the word 'Nexus' (or just searching
here) and you'll want Nexus Mod Manager to install them, and LOOT to put them in the correct load order.