I can come up with five quest around the Imperial City you can do quite easily since you already have the skeleton key and two of them you definitely need that *special* key to finish those quests. Like someone said earlier you can explore the game world for awhile, because I know one http://www.uesp.net/wiki/Oblivion:Master_Training quest you really need to explore the map possible two quests.
Imperial Corruption, The Order of the Virtuous Blood, Unfriendly Competition (inside the Imperial City)
A Venerable Vintage, Go Fish (around Weye)
Why not to get out to the roads and fight any bandits that shows up or they try to rob you.