One of the most distinctive things seen in Boulder are the formations. They are fantastic and the hikes are very nice. 

A suggestion is to get a trail map ahead of time and try to follow it, otherwise you can get pretty turned around between where you wish to walk and where you really do. 

Boulder is a great place, and the flatirons are wonderful. Definitely worth a day there at least an afternoon. Take a picnic lunch and have it there or in one of the beautiful boulder parks.