Boulder is about 5300 feet in elevation and fall is very dry. The majority of people who visit are fine. You WILL need to drink much more water than at sea level. Alcohol tolerance is lower. Keep water, snacks, and ibuprofen with you.
Farmer's markets are on Saturday morning in downtown Boulder (8am - 2pm, 13th street) and downtown Louisville (9am - 1pm, Front street). National Center for Atmospheric Research (NCAR) has a cool museum if you're interested in weather and climate science. Also a trailhead. Pearl Street in downtown Boulder is the place to be for people watching, restaurants, shops, and street performers.
For a nice easy walk, try Boulder Creek Path (downtown Boulder, goes up Boulder Canyon), Bobolink trail, Walden Ponds wildlife preserve. For those that want a pretty workout try these very local hikes (will be crowded, but crowded for a reason): Royal Arch or Flatirons from Chautauqua; Mount Sanitas; or any hike from the NCAR trailhead or South Mesa trailhead.
If you have a rental car, we recommend a drive along the peak-to-peak highway for aspen leaf peeping and some time in the mountains! Drive up Boulder Canyon to Nederland, a small town about 40 minutes away. Continue along highway 119 toward Ward, and return to Boulder down Left Hand Canyon. There are many places to pull off, and easy walks and hikes along the way (Mud Lake is easiest). If you only have a few hours, you should probably head back down at Ward, but if you have extra time, you can continue along the highway all the way to Estes Park, the famed mountain town that is the home of the historic Stanley Hotel and the east entrance to Rocky Mountain National Park! You will need a timed entry reservation if going to RMNP. For the really adventurous, make a timed entry reservation for Brainard Lake Recreation Area in Ward; conditions may be more like late fall/early winter, but the views are stunning.