Obviously this is a "good" character (0 infamy) so your "logical" options would be the "good" questlines (Mages Guild, FightersGuild, Arena, KOTN), and all of the small "please help" quests, which could keep you busy for a long time.
Your character at level 18 has done 55 quests. I have a somewhat similar character, at about the same level (17) who has done 117 quests. This is a "good" character like yours, who has not touched the Thieves Guild, the DB, or most of the Daedric Shrines. There's obviously a lot left to do!
Some of the "minor" quests, like the one starting in Chorrol, involving the twins, are really good story-lines, as well as giving good rewards.
If you have the KOTN add-on, I'd suggest you do it. It will take you all over the map, and you'll have opportunities to talk to lots of people, and pick up more quests as you go. If you're not a member of the Mages Guild and/or the Fighters Guild, I suggest joining both eventually, and running through their quest-lines at least once. They're worth doing; the story-lines for both of those guilds are almost as involved as the Main Quest. Likewise, the Arena is worth going through at least once, especially for the story-line that develops towards the end.