I would personally stick with the "This is what I'm going to tell you about" approach (people don't like having to figure out what it is you're trying to talk about). However, if you don't want to do that, you're going to have to come up with a creative angle on the trip.
For example, say you wanted the angle to be why the trip made you want to participate more actively in our government. Then you could start off your talk with a few reasons why citizen participation is important to our system of government. After that, PLEASE tell your audience what you're about to talk about, like "... with that in mind, I'm going to share with you my experiences on our 8th grade trip to Washington DC." Then, for everything you talk about, like visiting the Lincoln memorial, add a sentence about what that monument or whatever had to do with citizen participation. For example, if you visited the Vietnam War memorial, you could say something about how it is important to have a right to vote if you could be sent to war because many of the young men who died in Vietnam were not eligible to vote until after the constitution was changed.
Hope this helps!