Realistic armor weights, and realistic weapon weighting, (you won't need ridiculous amounts of Strength just to carry ONE set of armor or a standard load out of weapons at higher levels)
Realistic gem and pelt pricing (makes vendor treasure actually worth money, srsly a flawless diamond is the size of your fist and you get a lousy 100 septims for it?)
Archery rebalanced (Makes archery work more like archery rather than like you're using a large slingshot and darts.)
If you have a newer computer or a high res monitor get Dark UId DarN or Darnified UI (It gets rid of that awful Xbox interface for you)
Streamline memory manager (excessive amounts of memory is no excuse to let a game manage what it uses poorly.)
Oblivion Stutter Remover (Only if you don't have the ultimate gaming PC)
Those are the bare minimum I would suggest. I got all of them at Nexusmods. You'll have a relatively unchanged but efficient vanilla experience. Try browsing some of the mod sites for new content. --- Load the Spaceship with the rocket fuel, LOAD IT WITH THE WORDS!
General Overhaul: Qarl's Texture Pack 3 (alternatively QTP3 Redimized / Reduced / Redimized and Reduced if your PC can't handle the full QTP3; Redimized runs smoother and doesn't sacrifice much visual quality) and Bomret's Texture Pack (this covers SI)
Caves: Ampol's Cavern Retexture
Flora: Enhanced Vegetation + Improved Trees and Flora + Improved Trees and Flora 2 + Better Nirnroot
Miscellaneous: Better Staffs Texture + Improved Fruits, Vegetables, and Meats + Kafei's Better Amulet/Ring Replacers + VA's Better Gold + Book Jackets (Oblivion and KotN)
If you want your water to look better, install OBGE and OBGE Liquid Water.
The best weather mod, IMO, is All Natural. You can also install Natural Environments, but only enable the Natural Habitats component of it (Vegetation, Weather, and Water are already covered).
For better and more immersive sound: Better Bell Sounds + OSSO + Sounds of Cyrodiil + Symphony of Violence + Storms and Sound