Balboa Park. Wow. I haven't been there since high school, when we used to practice and play our lacrosse games there.
In grad school, my classmates and I used to rollerblade around the Rose Bowl (I think the loop around the parking lots/golf course is about 1.5 miles) and along the bike path from Santa Monica down to (almost) Palos Verdes. Guys also used to play pick-up street hockey in the beachfront parking lots in Venice (at least, during the week they did). Of course, it's quite a bit more scenic at the beach.
Esplanade in Boston is also nice, as is Memorial Drive over in front of Harvard when they close it to vehicular traffic on Sundays during the summer. I'm impressed you actually used 'blades as transportation in Boston, though, as the streets and sidewalks are so not-smooth.