san Fierro is fun. I've owned this game more than 4 times and the furthest I've ever gotten in this game is the flight school missions(no wonder I quit) but i'm back at it and i'm in san Fierro again. the great thing about the 3d era is that you always find something new when you come back. I did not know about the boat school or that passing driving school, valet, boating school and bike courier will gain you assets and you witll be able to collect money from them.
Make sure you complete the full main story of San Andreas, its worth it, the last quarter of the game goes onto to a rollercoaster finish and wild finale. I don't blame you for giving up on the flight school part though, when I first played the game I was stuck on it for a very long time, to the point I nearly abandoned it for a bit. But honestly just finish it, you will like it even more. You're right about new discoveries and SA is so full of that for me - when I played it again last year for only the second time in my life, I discovered for the first time that CJ could own a car sales business, run a pimping ring, play pool for cash, could date women in other cities (apart from LV), I never knew all of this at the start. I didn't even remember that SA even had strip clubs when I first played SA, can you believe that ? It's just a testament to the huge size and variety offered by this game to explore even 12 years after it was released.