I can't speak to the places you've narrowed down, but I was born and raised in Colorado and would recommend giving some other spots a try! Boulder is. . . meh. ;)

I'm not sure what your budget is, but here are some places you should both look into:

Aspen, Vail, Breckenridge, Telluride, and Winterpark (I clustered these together because they remind me of Boulder except with better outdoor stuff).

Colorado Springs - it's not the greatest but it's got dining, etc. and there's cool outdoor stuff nearby like Pikes Peak, Garden of the Gods, etc.

Estes Park is a neat town and Rocky Mountain National Park is right there and has some incredible trails and awesome scenery. Trail Ridge Road is a great experience, just be careful - altitude sickness is not a fun time!

Glenwood Springs has lots of activities.

If you're into casinos, Blackhawk has quite a few and some decent hotels, etc.

Denver isn't a bad spot. It's got plenty to do, decent people, and isn't far at all from all sorts of outdoor activities. However, traffic going towards the mountains is generally absolute shit, especially on the weekends!

Golden and Castle Rock are okay too, close enough to hiking and stuff like that.

If you want to go south, you could check out the Million Dollar Highway. The scenery is fantastic, but you won't find much city life! There's also the Great Sand Dunes, Mesa Verde National Park, etc.

I know I'm leaving out a ton of options, but wanted to give you some other ideas outside of Boulder! Good luck with whichever state you choose, and I hope you both have a great vacation!