When I was REALLY into running I lived in Arlington, VA and worked in Georgetown....the canal is a great place. I also liked doing loops along the National Mall....the big circle from the Capitol to the Lincoln Mem. Had to dodge tourists and stoplights, but it was so much fun.
The "boardwalk" along the beach in San Diego is nice too.
Now I live in Vegas....nowhere good to run here. Plus, it is too damn hot.
I am there (Georgetown DC) every week doing a long run. I recently switched from the tow path to the capital crescent which is paved, but in the same area.
i live in stamford ct, and i like to run down on the beach. its not bad scenery wise, and theres a 1 mile circle path already outlined so it makes it easy to know how far ive gone