Your best bet to get some worthwhile "beginner level" experience & practice with other boaters around is probably to run the Golden whitewater park as often as possible. Then step into other runs when you have someone to go with. At Golden, even at relatively high flows, you just need to put in a solid forward stroke at the bottom of each drop and you probably won't flip, but if you do, you should be working on your rolls anyway, and if you fail to roll, swims aren't too bad there (or, at least, you won't find any friendlier place to swim in whitewater in the area)... you may have to chase your boat further during high water. I would park at the standard parking lot on 10th street across from Lions Park, hike upstream as far as you want, find a good put in, and get some practice. Repeat. There will almost always be other boaters running it, and you can meet other folks there if you're friendly.
Deckers is also good for beginners, as mentioned. Be sure to scout "the chutes" ahead of time and have a plan. Needless to say, its best to not go alone, post on the Trip Planner forum to find when others are going.
Filter plant run on the Poudre in Fort Collins is also a standard, fun beginner run if you don't mind driving about 70-90 minutes. Milk run (and eventually Browns) on the Arkansas are another weekend option. The Colorado from Grizzly Creek through Glenwood Springs and Cemetery Run on the Roaring Fork are good beginner options in Glenwood Springs (~2.5 hours from Boulder).
Colorado Whitewater (coloradowhitewater.org) does a several river cruises and weekends a year, some of which are beginner friendly. Definitely worth attending if possible, you'll meet people to boat with and learn from others. You missed the training camp last weekend, but there's still several events through the summer.