I got to level 27 without repeating a mission, grinding, doing a flashpoint, or doing PVP. Then I wanted to take a break from story missions and started doing PVP.
On each planet I would go through and do each mission (heroics included), find each datacron, location, race, etc. Basically I did all there was to do on each planet as I progressed. I'm now part way done on Tatooine.