You can have a good number of nice mods without getting ahold of Shivering Isles. The only mod I can think of that I've using that *needs* SI would be Fearsome Magicka, and while it adds some nice new spells, its hardly essential.
Try OOO and see if you like it. I've been hesitant to myself, but a huge amount of people in the mod forum seem to see it as essential, so it's probably fun
Find a post by CorePC and look at everything in his signature (MMM, Tamrielic travelers etc)
I tend to view MMM, COBL, and Midas Magic all as essential to a TES4 installation.
(
MMM adds monsters and loot and crafting of some weapons/armor
COBL adds a bunch of stuff too varied to enumerate here
Midas Magic adds lots of new spells, as well as a variety of quests in which to use them
)
After that its a case of seeing what you don't like in the vanilla behavior and how you want to change it.
I've found governing attributes is pretty nice and edits your skills in a way thats non-destructive.
http://www.tesnexus.com/downloads/file.php?id=24281
If you enjoy alchemy try Grandmaster of Alchemy and/or alchemy advanced
http://www.tesnexus.com/downloads/file.php?id=16441
http://www.tesnexus.com/downloads/file.php?id=25226
If you think the UI is huge and made for consoles and want a different approach, try Darnified UI
http://www.tesnexus.com/downloads/file.php?id=10763
I personally like the mod Old Friends, it makes items better as you use them, thus letting you keep hold a sword you find early in the game and don't want to have to discard cause the new enemies are harder.
http://www.tesnexus.com/downloads/file.php?id=19067
The ayleid wells --each well you visit gives you a small permanent boost to magicka, thus a reason to find them
http://planetelderscrolls.gamespy.com/View.php?view=OblivionMods.Detail&id=435
vaults of cyrodiil --adds small loot-filled vaults in the various castles of the realm
http://www.tesnexus.com/downloads/file.php?id=9357
Shady Sam Plus --adds a new shadysam type person to each city
Advanced Magecraft --lets you do all sorts of neat stuff with spells and enchantments (could be easily abused, so avoid if you can't help yourself
)
Vampirism2 --a different take on how you are as a vampire
http://www.tesnexus.com/downloads/file.php?id=26914
Landmarks w/ wells --gives map markers for ayleid wells, and stones.
Medic! --lets you give potions to NPCs, so all your escort missions arn't so heartpounding
for more quests, added items etc
The Lost Spires --excellent mod with new items/creatures/dungeons
http://www.lostspires.com/
The Ayleid Steps --much newer, but still well-made mod with new dungeons to explore, and new items courtesy of COBL
http://www.tesnexus.com/downloads/file.php?id=16316
and thats pretty much my whole load order
I hope that helped...