This thread is old but deserves an answer anyway. Lots more people buttboating in CO these days with the same question.
Best thing you can do is to get Stafford and McCutcheon’s ‘Whitewater of the Southern Rockies’. Great beta, stunning pics, hundreds of runs in NM, AZ, CO, UT, and WY. Not packrafting-centric but we’re adults–we can extrapolate.
IME thus far:
Northgate Canyon on the North Platte is a fun run. Did it as an overnighter with fishing gear and that was about right. Class III at almost any level. More about the scenery than the whitewater.
Many sections of the Poudre above Fort Collins are suitable–see above text.
Clear Creek into Golden has everything from II+ to V. The after-work run for most Front Range peeps. Almost entirely roadside means it’s easy to lap and learn and meet fellow paddlers on.
Arkansas from south of Leadville to Canon City has hundreds of rapids of every flavor and lots of road access points to allow you to bite off as little or as much as suits you. BV and Salida have play parks that are great, friendly places to learn in a ~controlled setting. Sections like Fractions and Browns are great for fast learners to dive right into, or timid second-season boaters to start to spread their wings on.
Waterton Canyon on the South Platte is an excellent spot to learn by sessioning III and III+ rapids. The run is only one mile long but has a wide, flat streamside trail that you hike back up when you’re done. Suitable flows can be found 9-10 months of the year. Nearby Deckers section caters to true beginners.
Gunnison has a whitewater park and a town run that are both great places to learn and progress. Taylor Canyon NE of Gunnison is, IMO, one of the most fun III+ runs in the state. All roadside so you can bail if needed, but interesting/continuous enough and in such a pretty canyon that you don’t often notice the road is there. Probably past the limit of what anyone from ~Denver would want to drive to for a day run.
Haven’t yet run the Blue River out of Summit but I’m told it’s mostly mellow class II–better as a fishing float than for any sort of whitewater thrills.
This doesn’t scratch the surface of what CO has to offer. Get the book and start there, cross reference the beta within with the flows found here:
http://waterdata.usgs.gov/co/nwis/current/?type=flow&group_key=huc_cd
And you’re well on your way.
MC