The Prometheus Blu-Ray's Extras menu reveals what the Engineers were doing: seeding Earth with intelligent life:
Scene 7: Arrival of the Engineers
The Engineers touch down on prehistoric Earth for a sacred ritual devoted to the seeding of Intelligent life. Whereas the Theatrical version of the scene is devoted to the Lone Sacrifice Engineer, this early cut features several of his comrades, including the Elder Engineer. The ceremony was filmed with dialogue but unsatisfied with the result, Ridley Scott removed the dialogue entirely.
As it was mentioned, there was already plant life on the planet. This can been seen from the areal shots.
