In this article the author talks about one of their past experiences in day hiking and what you need to know if you decide to go day hiking yourself. The author gives many example of what you can bring on a day hike so that you are well-prepared for any situation.
On one of the author's first day hikes he remembers having seen many people on the trails who were ill-prepared. One couple in particular hadn't even brought any water with them. He continues by saying that water is the most essential thing when going on a day hike as without it dehydration, along with other things, can become a problem. He also gives lots of specific things to bring in your day pack so that you don't go ill-equipped or unprepared.
I go hiking from time to time, when it's not the winter of course, so I found this article provided a lot of good advice even though I don't usually go for full day hikes. In general, I thought this article was a good read that provided me with some good tips in the form of several detailed examples.
Day Hiking Essentials, Outdoor Adventure Canada