I finished Dragon Age 2 and I have a few questions?

To get to level 20, just keep going to places like darktown, and the free marches. There will sometimes be random enemy encounters there. Also at night walk around lowtown and hightown for fights to level up. The high dragon is at the bone pit. There is a side mission at act 3 that will ask you to kill it. Since you've finished the game, you can start a new one as a different class (mage, rogue, warrior), you can have a romance with a different character, choose different dialogues, and make different decisions during certain quests to change the outcome... Trust me there is a lot that can be done. It is not as good as origins, but it's still a great game.