I would avoid the Benadryl - there are a good chunk of kids who actually get over the top, crazy hyper as a reaction to the medication.
In terms of being very routine-oriented, I would think two things. First, you will be way out of routine on vacation and your daughter might be okay with a total change. Kids are surprisingly adaptive if we let be (and not freak out about it). Second, there are parts that might be rough (really when is any kid an angle for days in a row) just know that there will be rougher times, maybe nap time for you, but there will also be great times to make up for it.
Our first trip with my son, who is a terrible sleeper but great out in public, he was a nightmare at every restaurant we went to for the first two days but slept with no issues. Best wishes for a fun, relaxing trip!