Getting enough sleep. Drinking enough water. Getting out in the sunshine at least 15 minutes a day and feeling the breeze on your face (if you think about it, a lot of people never really get out of the house except to walk to their car and back).
I love when the winter starts turning into spring, which in some places really means when the winter starts turning into the summer. The air starts turning and you can smell the season changing.
Sometimes TV marathons make me happy, but generally when they are on DVD and/or when I am taking a coveted off day. In general, watching TV makes my brainwaves scattered and makes me feel distracted, especially when I have a lot of "free time" and could be more productive.