I'm a high school senior (honors & Advanced Placement) English teacher, and it took me some time to understand the delicate balance of what being a senior in high school means. While I agree with the previous postings regarding understanding your objectives, I do not believe that your objectives should be the only force to drive the instruction - I believe students should be the main factor in creating objectives, which will then lead to instruction. Those objectives need to be shaped around the balance - "young adult" - a high school senior is the true embodiment of what a "young adult" is; they are mentally preparing for college, while they are still in high school -- they are both "kid" and "grown-up."
Through understanding this, you must make activities that stimulate the "kid" side, while preparing them for the "grown-up" aspect. I found that the best in-class responses from seniors were through debates and whole group discussions over controversial topics. Whether you teach English, History, etc. - every subject poses its controversies, and senior year is, in my opinion, the best time to bring those to light.
I also find that seniors will respond to large projects that require them to explore outside of the school building. This appeals to their desire for freedom while still adhering to structure and guidelines. For example, I asked my students to visit the Met (The Metropolitan Museum of Art - New York, NY) and choose a favorite piece of art (painting, sculpture, artifact, etc.). They were required to give a voice to that art (we were discussing the use of dialogue with Pride and Prejudice), and explain which character would be most drawn to that art and what textual proof do we have of that.