Extreme Stunt: Ruler of the Clouds
Extreme Location: Cloud Ruler Temple
Extreme Gear: Acrobatics stat of 40+, Potion of Fortify Health, Potion of Fortify Agility
If you've followed the main quest a fair amount, you'll gain early access to the high-altitude sanctuary at Cloud Ruler Temple. Situated at the pinnacle of a mountain just north of Bruma, Cloud Ruler is the perfect place to train your fleet feet and extreme diving ability. (Don't forget to watch the two soldiers training out front during the day - you'll get a Blade and Block stat boost there, too).
Can you jump high enough to gain access to the roofs of the two small guard towers overlooking Bruma? If so, buff up your health with a Potion of Fortify Health or two and take a flying leap into the chasm in front of the temple.
It's about a 200 foot drop. Did you survive? If you did, run back up to through the temple (while healing yourself and increasing your Restoration stat) and repeat. The cliff face in front of Cloud Ruler is slanted outwards and double-black-diamond-steep, making it a perfect training ground for extreme stunts. The faster you run and longer you jump, the further out you'll go and farther you'll drop.
If you don't survive, you'll get a nice demonstration of Oblivion's incredibly realistic physics - which is to say, it's morbid fun to watch as your fancy armor-covered carcass careens noisily down the slope. Dial back your jump bit by bit until you can survive it, then repeat the jump until you hit a higher level. It'll happen fast - big drops equal big gains in your Acrobatics skill.
If you can't even gain access to the roof, don't sweat it until later - you wouldn't survive the drop, anyway. Your Acrobatics stat not only dictates how high and far you can jump, but how much damage you'll take from distant drops.