There are many excellent options for boating that time of the year.
I suggest you consider the Cache la Poudre river as a good destination for at least one or two of your days. The base of the canyon is an hour and a half north of Denver. The Poudre has numerous runs with long stretches of III/IV both above and below the narrows section. Can be busy on the weekends.
The Arkansas is also an excellent choice with the Numbers, Brown's canyon and the Royal Gorge being the more popular runs. Avoid Brown's on the weekend when it is paved with rafts.
Glenwood Springs would be another good base camp for the Colorado, Roaring Fork and Crystal rivers.
I would suggest getting a copy of Colorado Rivers and Creeks vol 2 if possible. It's been out-of-print for some time but rumor has it that you can still buy it directly from the authors.
The "CRC 2 Buy" item under the site menu has the address and price.