I'm still sort of new to some aspects of Oblivion. I've completed all of the factions and have all of my attributes maxed at 100, something like 1M septims worth of items stuffed in clams, and almost all of my skills at 100. I only recently started making my own spells and enchanting my items. For my first item, I enchanted a ring to give a constant effect of fortify strength +10 using a Grand soul gem (Level: Grand.) Is that the extent of it though? Can't I put multiple enchantments on one item (akin to quest rewards, such as the grey cowl of nocturnal)? I'd like to go super overboard with my enchantments but it seems sort of limited. For example, I really like the idea of a constant effect of Reflect Spell(100%) but I can't enchant an item with that as a constant effect. I'm playing on the Xbox so mods are of the question.

Yep, one enchantment per item is the limit on custom made magical gear. They put a limit on it for balance, no doubt. I feel you though, i too wish you could add at least two half-powered enchantments per item or something....

Just one enchantment with constant effect, weapons can have multiple, you could make multiple weaker in Morrowind, but it was seldom cost effective, yes it was exceptions for weak effects like water breathing and walking so you could combine them
Weapons can have multiple effects and you can make lots of fun; soul trap 1 second, fire damage 15, weakness to fire 100 in three seconds, and second hit does double fire damage if you hit after before the weakness times out. Same effect works for spells

For fortifying attributes, Fatigue, Health or Magicka, the greatest enchantment will come from using a sigil stone that you encounter after you are at least level 17 (Transcendant Sigil Stones). Below level 17 you get Ascendant, Latent, Subjacent or Descendant Sigil Stones which are weaker.

http://www.uesp.net/wiki/Oblivion:Sigil_Stone

http://www.uesp.net/wiki/Oblivion:Enchanting#Enchanting_by_the_Altar.28s.29

Using a Grand Soul Gem at an Enchanting Altar, you can enchant wearable items (e.g., Shield + Helmet + Cuirass + Greaves + Gloves or Wrist Iron + Boots + 2 Rings + Necklace) to provide the following constant effects:

Fortify Attribute 10
Fortify Fatigue 25
Fortify Health 10
Fortify Magicka 24

Using a Transcendant Sigil Stone (no altar needed), you can enchant wearable items to provide the following constant effects:

Fortify Attribute 12
Fortify Fatigue 50
Fortify Health 30
Fortify Magicka 50

It makes a huge impact on Magicka for example if you enchant all 9 wearable items to provide +50 magicka, for a total boost of constant +450 Magicka)

Why not go questing for high-end enchanted jewelery? Caves that have only animals in them tend to yield this kind of treasure. My first thought was the Ring of Perfection, which does the +10 Fortify Strength, in addition to much, much more. Unfortunately, it is random, but at your level (which I assume to be over 25), you should be able to find it in about three caves or less.