You could do this a number of different ways. If your field trips are very regular, or part of an organized co-op, you might want to create a Lesson Plan just for these trips.
If your trips are more eclectic, you might want to create a journal just for field trips. The advantage here is that you can write about them and include pictures for each entry as they happen.
I would not add them as a holiday, because field trip days can still be considered school days.