One simple exercise from the field of Positive Psychology that has been shown to increase happiness is the "three good things" exercise.
Here is the exercise:
Every night for the next week, before bed, write down three good things that happened that day and why they happened.
"My husband made me a cup of coffee this morning" Why did this good thing happen to me? "Because he cares about me".
"I received positive feedback from my boss on a report I did" Why did this good thing happen to me? "Because I worked really hard on the report".
"I discovered a beautiful garden near my house" Why did this good thing happen to me? "Because I made the effort to get off the couch and go for a walk".
The three good things exercise is founded in the idea that we spend too much time in our lives focusing on the things that have gone wrong, and not enough time focusing on what has gone right. Such negative overthinking can place us at risk for developing depression and anxiety, and can slow down recovery if you are experiencing those illnesses.
Studies have shown the three good things exercise produces lasting increases in happiness for up to six months.
Give it a go and notice what happens to your mood!