Colorado loves craft beer, and Denver is lucky enough to host the annual Great American Beer Festival at the end of September or beginning of October. Over three days more than 50,000 locals and visitors pack into the Convention Center to sample as many 1-ounce tasters of the nation's greatest brews as they can handle.
This is easily the biggest and most diverse beer festival in the world- if you consider yourself a beer fan, you owe it yourself to go at least once in your life. Unfortunately entry is rather expensive (the ticket price includes as many samples as you care to drink), and the event sells out months ahead of time so planning ahead is required.
If you can't make the festival, it's still a great time to be here. For ten days the whole town joins in the fun with innumerable smaller events.