Just a thought - We both have Galaxy IIIs and the batteries usually last all day even using Waze, and the internet all day long. On our trip to Universal Studios over Christmas holiday we made sure to charge both phone over night and checked they were full before we left for the park the first morning. Without changing a single setting, BOTH phone were down to 25% in less than 2 hours at the park. I was furious because I really love taking pictures, and it was our first time to see Hogwarts - I am a huge Harry Potter fan and I'm still holding out for my Hogwarts letter!!. Anywho, I was talking with my mom later that evening and she thought it was probably searching for a signal that drained the battery. Sure enough, the next day we left the phone in Airplane mode except for a few times we wanted to make a call or check the wait times with Undercover Tourists and our batteries lasted ALL day long only reaching about 75% by the time we left at close.
I know this isn't an option for some because they need to be reachable but it does help to be mindful and turn it off when you really don't need it.