Would you be here in September or October?
There are several city fall events both months.
Here are a few of the larger events:
Art in the Park is in September. I believe this is the largest art vendor event in the Northwest.
Boise Art Museum - Art in the Park
Hyde Park Street Fair is also in September.
2011 Boise Hyde Park Street Fair @ Boise Online Mall
Every Saturday is the Farmers Market in Downtown.
http://www.capitalcitypublicmarket.com/
I know there are several events throughout October, here is a link for Fall For Boise which is a ton of Fall city events, including beer events throughout September and October.
FallForBoise.com - About