Boulder is 5500 above sea level, roughly. If you don't at first, you may feel affects from the thin/dry air. Be sure to drink LOTS of water, wear sunscreen (more than you normally would), and BRING YOUR SUNGLASSES! The sun can be very bright and very strong. Over-hydrating is a great idea to keep you from feeling any negative side effects of the dry climate. In short: Bring you sunscreen, your hat, your sunglasses, and a good, large water bottle!
The wedding ceremony will take place at the St. Julien Hotel and the reception will be at The Dushanbe Teahouse, which, if you plug them into Google Maps, you'll see is just a couple blocks over and a couple blocks down from the St. Julien! Feel free to take the walk over the bridge and past the park!
That depends on what you intend to do! Boulder (and Colorado in general) is a very casual place. Bring comfortable walking (or/and hiking) shoes and versatile clothing. The temperature ranges from 40-80 degrees in October. Expect it to be cooler in the morning and warm in the afternoon, though weather can change on a dime, sometimes changing 40 degrees in a matter of minutes! . Be ready to layer if you want to be consistently comfortable. Even in the fanciest restaurant in Boulder, you'll find hiking shoes and sunshirts at any time of day.
It depends! if you stay at one of the recommended hotels, there is little need for a rental car. You will be able to walk to the wedding and reception, and the brunch the next morning. You're just a street off of Pearl Street, which is a lovely mainstreet, and short Lyft ride from many foothills paths and trails. The rideshares from Denver Airport to Boulder are generally ~$75 and if you don't want to rent a car - you won't need to. However, if you want to do any side trips to Rocky Mountain National Park (50 minutes away) or Denver (35 minutes away) or Colorado Springs/Garden of the Gods (1.5 hours away), or Breckenridge (2 hours), you may want to have a car for convenience. Parking in Boulder is fairly easy and much of the public parking is free.
Rocky. Mountain. National. Park. Estes Park, which is the gateway to the Rockies, is only 50 minutes from Boulder (and it's a jaw-dropping drive!) and boasts an adorable main street with old-fashioned candy shops, burger joints, and chocolate shops. Rocky Mountain National Park is a wonderful place to spend a day with many hiking trails, ranging from easy to majorly difficult, and gorgeous mountain lakes and views to marvel at. If you have extra time - do that. We cannot think of a better way to spend time (than maybe attending a wonderful wedding, of course!)