I'm sure this is a no-brainer, but if you're in the Denver/Fort Collins area and haven't been to New Belgium I would highly recommend it. 90 minute tour with good samples. Get to learn a lot about how they got started and how they do what they do. Highlight of the tour was getting to go into the sour aging room, which they refer to as the Foeder Forest and drink La Folie next to the barrels it was aged in. Room smells like red wine and whiskey. It was pretty incredible. Not surprising based on their distribution footprint, their bottling line is MASSIVE. Pretty nice if your a tech/equipment geek. Plus you get to taste samples of whatever they are bottling at that moment, so you're tasting 5 minute old beer.
One word of caution, you do have to book the tour in advance, probably well in advance for Saturdays.
Also, side response to the lower post about Denver spots, replying way too late for his trip, but definitely check out Hogshead if you're a fan of English style beers. They had about 6 on cask and 5-6 on CO2. The ESB was incredible, and the English IPA was a nice subtle, earthy, nice change of pace compared to NW style (which I love, don't get me wrong). We almost skipped it since it was the last place on our second day but I'm glad we didn't. It ended up as some of the best stuff we had that day.