Writing isn't exactly everybody's thing. However, writing about your day should be. The more adult version of a diary would be a "journal". Diary-keeping, journal-keeping, whatever you want to call it, should be in everyone's daily routine. You may ask why, but the answer is pretty obvious: it's good for you!
How exactly is keeping a diary good for you? It gives you a way to talk about your problems. We all have day-to-day problems, whether they're big or small. You shouldn't keep anything bottled up, no matter the issue. Telling someone about said problems may not always be the comfortable option. Although writing them out in a notebook of your choice can be. Essentially, you would write in a diary everyday. Not everyone has time for that or remembers. If you do remember, though, make sure you do it! It has the potential to make you feel so much better. It's a great way to end the night, or maybe even start your morning.
You can pretty much keep whatever you want in it. Maybe it's gossip, what you ate today, or something about your significant other. You don't only have to write your problems in it. It's, honestly, whatever you want! You can even look back at older entries to see how you were feeling a certain day. They are more than just something you write your thoughts in, they're also just a lot of fun!
It may seem quite childish. Keeping a diary is totally a elementary school girl thing. Except, it's really not! I suppose "keeping a diary" is a little off putting for some people. However, that's what it is! I know that's what I call mine. If you want to call it "journaling" then so be it! We are all different and prefer certain things. Something we all have in common though are thoughts, thoughts that need to be shared with another person or yourself.