A day hike is any walking tour where you don’t take all your gear with you, but instead come back to your starting point in the evening. It’s important that you take the equipment you need for one day (without an overnight stay) with you.
You should always have these with you, too
Optional (depending on the season and the tour)
If there’s space left in your backpack
What might be the biggest advantage of day hikes: mistakes in equipment planning are only a nuisance for one day and can be remedied relatively easily. You should still definitely have a few essentials with you that are necessary for the tour. Everything else then falls into the category of “personal preference” and “habits”.