Then there are quest rewards. One major quest reward gives XP equal to that for one encounter. It's pretty easy to say that each adventure gives one of those.
Let's not forget skill challenges. They grant XP just like an encounter.
So if a DM wanted to level up the party rather quickly, it wouldn't be hard for him to design adventures for that. Let's say he wants to use the 5-step method, which actually has four legit encounters since the last step is the "aftermath". It might run thusly:
- Recieve a Quest in town - eventually grants XP for one encounter.
- The PC:s find their way to the dungeon through the wilderness - skill challenge, XP for one encounter.
- Entrance and First Rooms - one skill challenge, two normal-level fights. XP for three encounters.
- Setback - major fight. XP for one-and-a-half encounters.
- The PC:s work to mitigate the setback and get back in gear to take on the final fight. Skill challenge, XP for one encounter.
- Climax fight - major fight. XP for one-and-a-half encounter.
- Cleanup - skill challenge as the PC:s investigate what to do next. XP for one encounter.
- Total: XP for ten encounters.
There you go, new level after only four fights (and you can make that three by turning one of the normal-level fights into one more skill challenge in a puzzle-heavy dungeon). Who said D&D was only about fighting?