I'll have to confirm with my Japanese colleagues why Marunouchi OL are reknowned. Had mentioned to a colleague that my GF is working there and he just smiled knowingly and said, "Ahso Marunouchi OL." So I have no clue if he meant anything by that or not. Mysterious Japanese smiles.
For, ahem, sightseeing I find Omote-Sando area quite nice with lots of well dressed attractive ladies. For something different, Shibuya has all those orange tanned and blonde haired yamamba girls and Harajuku on the weekends is known by all I would assume.