A lot of the historic buildings are on Pearl Street in Boulder's downtown.  There's a four-block pedestrian mall on Pearl Street between 11th and 15th Streets that features a lot of historic buildings, and there are plaques that provide historic interpretation.  The mall (and other sections of Pearl Street) also has several interesting shops, street performers, outdoor concerts, and the like.  It was enough to keep a tourist like me entertained.  If you ever go back there again, check it out.   (Elkspeak)