I'm at my best when I'm challenging myself to be better, pushing what I understand and think possible, and imagining creative ways to solve problems. I love the fact that I get to go to work/school every day and work on problems that no one has ever thought of or solved, and I get to push the boundaries of knowledge.
When I'm not pushing my limits (you can't go 100% 100% of the time), I like to relax with friends, often while playing a board game or card game. I organize a board game night for computer science grad students. I love seeing students who typically have difficulty socializing sitting around munching on snacks, and fitting in while playing their favorite games.