First of all, I would look around and see what young adults are into. They like facebook, you tube, video games, facebook games, clubs at school, TV shows, hobbies. (I have many friends within the ages 15-25 and this is what I consider young adult: high school to college level education. . .)
Female, especially in high school, seem to be interested drama, thrillers, things to talk and gossip about. Younger men seem to enjoy sports, and girls.
I would defiantly make youtube features. There are more young adults that view youtube than anyone I know. If you know any young adults, using them for the youtube pieces, I think would work best. Who can reach out to young adults better than other young adults?
If you really feel lost, then send a poll around to see what young adults on facebook are currently interested in.
What type of books do you read?
What type of TV show/movies do you watch?
What pulls you in to these types of genre?
Video blogs . . . I would get a young adult that is similar to your character and have them read from your book on you tube or have them talk about themselves and the story. Making your characters come to life really pulls people in. (If you are low on money, some out going young adults just like the attention and some might do it for a free meal or movie.)
Example: One of the YA book series I really like (it’s not Twilight) it’s called Pretty Little Liars by Sarah Shephard. http://www.amazon.com/-/e/B001IGNMHS?pf_rd_p=1347712082&pf_rd_s=center-2&pf_rd_t=1401&pf_rd_i=1000773601&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=0W4FF2FP50CT4DWX2W50
It is both a book series and a TV series. There is a lot of suspense, drama and thrilling moments. (I love thriller, suspense, drama and horror, so this series just pulls me in.)