Yeah, like many of you, playing 'mue and on my Dreamcast were the big things; read the entirety of Code: Veronica's TV Tropes entries this morning and it brought me back to my first (and, well, only) playthrough of my favourite RE game and how much I enjoyed all of it's nuances and plot (being my first RE game, it was a wonderful experience).
So gaming on the DC was a huge thing for me.
Playing a bunch of sports too, both in school and the community (plus with my friends, after school and all that), it was a time when I was quite active (yet didn't drop weight). It was a time before the internet and cell phones exploded, thus it was great to get to know people and spend time with people in person; probably the biggest thing from a social standpoint, of the 2000s.
Also defined my music tastes with heavy listening (almost 3 years and 2 years exclusively, respectively) of the Who and Limp Bizkit, so that was a fun time for me, discovering an old band and a new band that still had releases.
The 2000s also crushed my boyhood dreams, profession-wise (though this wasn't a bad thing), as it taught me that to attain what I wished to attain, required a bunch of game-playing, wealth and connections that I couldn't have or build up at that point in my life and that it taught me to become adaptable and to expand outside the box, both my lifestyle and my way of thinking.
These were the years I discovered myself and where I laid the foundation for myself today.
