Learn to meditate. This helps me out a bunch when I'm strung up, and I can even teach you in a single paragraph, if you want. You can read up on it, but I've read some guides online and found them vague and unhelpful.
Other than that, there's a list:
Exercise. I recommend cardio, but strength exercises work well, too. Running a mile after a stressful day can do wonders.
Nature. Visit some natural place, like a forest, park, or beach, and enjoy it. This can be mixed with exercising.
Eat. Munch on some healthy foods - Lots of fruits, vegetables, whole grains... And then have some ice cream or something. If you're exercising, it won't be a problem.
Sleep. Get at least 8 hours of sleep every night. Get used to a schedule, and try not to sleep too much into the morning. Light doesn't work well with sleep. If you're short on sleep, don't be afraid to take naps. Avoid stimulants like caffeine after lunch, and don't drink to help yourself fall asleep. Alcohol prevents you from entering a completely rested state.
Talk. Tell someone about your problems, and then...
Listen. Get sincerely invested in other people's lives. It helps you focus less on your life and encourages empathy, which makes us humans happy.
Relax. Take a bath. Read a book. Don't
try to have fun while you're doing stuff you enjoy. Just let the fun happen.
Create. Make something you can invest your emotions into. It doesn't matter what, but put your heart into it.
Meditate. Helps you focus, clears your mind, and refreshes you emotionally. It's pretty easy, too, if you take the time for it.
Bonus points if you can make a cool acronym from all that stuff.