Yeah so there's a reason Denver is called the Mile High City! It can take a couple of days to adjust, so take it slow initially. Lots and lots of water will help, maybe don't run a marathon when you land, limit your alcohol intake (if possible), and ibuprofen!!
Party clothes! Dress in your favorite party attire!
Casual! Dress up or dress down, whatever feels best to you!
For this one, I would not recommend RTD. You can either do a Bird or Lime e-bike (need either app to locate) and take a ~30 minute ride and cruise through breweries on the way. Or, take a Lyft or Uber.
Plenty of food and drinks! Come hungry and thirsty!!