Very early since Boulder is a smaller city attracting lots of tourists in the summer with a more limited selection of places to stay. View Travel section for specific details.
Just underwear and socks. Boulder is a very casual, outdoorsy town known for very hot, dry summers. Large majority of restaurants will not require any dress code.
We would recommend trying to find a hotel or AirBnB/VRBO close to Pearl St. in Boulder, if possible. Pearl St is central to shops, restaurants, and the shuttle location. Boulder is a very outdoorsy town, so feel free to pitch a tent anywhere.
Cocktail Attire: Wear cocktail dresses, dressy jumpsuits and jackets and slacks. Honestly, Boulderites are known to think Crocs with socks look cool.. so whatever you wear, you will probably look better than the local population.
There are many dispensaries located all around Boulder, including on Pearl St, so you can get that sticky icky.