These have been mentioned, but consider them additional votes:
Portland, OR
Sacramento, CA
San Antonio, TX
Omaha, NE
Minneapolis, MN
Indianapolis, IN
Reno, NV
Salt Lake City, UT
Some that you might consider small and others might consider medium:
Eugene, OR
Chattanooga, TN
Boise, ID (lots of neat places to see in the surrounding area, too)
Bellingham, WA (actually is small, but has feel of larger than it is)