The emperor is dead, you're a mage/fighter/thief, gates to Oblivion are being opened, and your doing all you can to avoid stopping them...I'm a bit of a ways into Oblivion, yet barely scratched the surface of the main quest, let alone all the other side quests and guilds...
My character is a level 14 breton with glowing eyebrows that wears light armor and a hood, is strong in magic but not so much with physical or archery. He's currently at Warlock rank in the Mages Guild, reached Apprentice rank in the Fighters Guild, and just joined the Thieves Guild.
I have completed several side quests (mostly regarding the Mage Guild), but I haven't touched the main quest past finding out the emporor's son is in the town with the Oblivion gate...
Today I chose to continue one of my several quest that I had already started, the one involving learning to summon a small monster (I think that's the result). The first step is to get a dark soul gem, and the only way I knew how is by placing a normal soul gem in an alter over night at a particular day of the week, but I didn't know when. So I left a gem in the alter and decided to just check back every few nights, and to kill time I looked around the area on foot.
I soon came across the Drakelowe farm, but only one around was who I assume is Ms. Drakelowe and she just told me to GTFO (might be a quest location later). In the same area though was an old mine and in it were about a dozen very well armed thieves...
After MANY restarts I managed to clear the caves, and while all the chests just had a max of 52 gold apiece, the equipment of the thieves fetched me nearly 8K and a full set of light mythril armor I kept. In the process I also leveled twice and advanced in nearly all my skills, even reaching apprentice in several.
So in the end I didn't advance in any quests, but I did some very productive grinding!