I went to New York City with my cousin when I was 10 and he was 12. We goofed around on the subway and ended up at the Natural History Museum, where we spent the entire day running through exhibits, naming all the dinosaurs and playing tag in the Hall of Biodiversity. We saw 3-D movies and live spiders. We found out what we'd weigh on Neptune. We had a fabulous time.
At the end of the day, my mom took us to my favorite diner in the city, where we all had cheeseburgers and milkshakes. It was the middle of the summer, and we had no responsibilities to get back to. We talked non-stop on the way home, and that was when I realized I'd just had the best day of my life. Nothing has surpassed it since.