I Had A Fun Day
When I was in high school, my English classes had us reflect on our days at the beginning of each class. At the time, it seemed like a pointless activity but that was simply because I was too young to realize the importance of self-reflection.
When you reflect upon your day (whether it is strictly just thinking about it or if you write about your day in a journal), it helps you analyze the things that happened to you. Did you have a fun day today? What made it fun? Are those the types of things that make you happy on a short-term scale or did it make you feel happy on an inner level? This also adds value to your experiences and may give you some new perspectives on the situations.
Reflection is a good habit to get into every night. Just find a nice quiet spot to sit and think about your day. You can do this in the bath, on the couch with a cup of tea, or on your porch while you are watching the sunset.