Consider, perhaps, an episode in your life in which you did not feel like doing something, but your parent(s) coerced you into the action, and later you had a wonderful experience. For instance, often teens do not wish to accompany their parents to places because they might be embarassed if friends see them engaged in a now-perceived "childish" activity such as camping. However, unexpected things happen when one camps sometimes, and the trip becomes exciting, entertaining, insightful, loving,.....
Or, perhaps, you have been compelled to work on a project with a fellow student whom you did not like, but after working with him/her, your perspective changes because of different things that have happened and you become friends after this enlightening experience of learning to understand someone.
I agree that the experience you choose does not have to be your greatest ever, but it needs to have meaning to you. Simple ideas that helped you make a decision, helped you understand an experience or someone in ways you hadn't considered before, or helped you change something about yourself whether physical or mental. I listened to a sermon in church which was a plea for people to help out at the jail. I joined the group which has changed much in my life where I get more than I give from the experience. You have people who have influenced you whether positively or negatively, and I'm sure you learned much from such an experience. Use an experience which shows something about YOU and how it affected you or those important to you. Small changes can lead to really important views of yourself and your place in the world.
You might right about a "first" or a "last"... The first day of a particularly interesting or important school year might make for an interesting essay that would naturally fit both the narrative mode and the descriptive mode.
Also, the first day at a new job or the last day of a school year, a summer camp, or a job might make for topics for an assignment like this.
Remember that you are telling a story (narrative mode) about yourself. I suggest choosing an event that was very meaningful in your life that you remember well. Discuss the people or events that had an impact on you. Use a lot of description and sensory detail to make the story come to life.
You are a high school senior, so you are likely to be seventeen or eighteen years old now, already with many experiences that you could write about. Since you are expected to reflect upon an experience, you will want to choose one that has made you think, some experience that has taught you something about yourself and/or others. Perhaps you have had a relationship with another that was a learning experience, for example, a friendship that had some difficulties. Maybe you have participated in a school activity that was a huge challenge to you, a sports team or a drama club. Your relationships with family could also be possibilities, an experience that taught you to have more respect for your parents or that provided you with some insight into a sibling. Remember that the experience you write about does not have to be an earthshaking one, just an experience that was important in your life, something that caused you to look at the world in a new way.