21. Summer vacations.
As much as I'm theoretically for longer school years, there is something great about having a summer vacation. It may be that greatness is built into my mind because I had summer vacations as a kid, so I now think summer=vacations, but I think it's something more inherent in summer: the warmth and sun and general longer-day-ish-ness and relaxation offered by the near-total lack of holidays to force you to spend time with your family all add up to something great. Summer is so synonymous with vacations that northerners like me wait until summer to take our vacation, and then we go some place warmer (i.e., more summer-ier).
By contrast, when I take vacations in the winter, they're... well, okay, I guess, because vacations are good, but the best winter vacation I've ever had was when we went to Mexico just after New Year's -- leaving Wisconsin to travel to summer. Otherwise, winter vacations just seem to be lacking. Who can say Christmas vacation from school was as good as summer vacation?
Maybe it's because when you go on summer vacation from school, it's long. And when you take a summer vacation as an adult, you start on summer, travel to more summer, and then come back to summer.
Other Best Things About Summer:
1. Ice cream.
2. Wearing a slightly damp bathing suit...
3. The smell of grilling in the air.
4. The streets always seem to be lined with American flags.
6. Sudden Thunderstorms.
7. Roller coasters.
8. Helium balloons.
12. Longer days.
16. Beach volleyball.
17. Entering an air-conditioned building after being out in the humidity for a while.
19. Carnival Midways (after dark).
20. Shirts that aren't, quite.