IMHO, in order of preference for August Rafting:
Arkansas River - Buena Vista, Salida, Royal Gorge. This long stretch of river has a wide variety of difficulty, so you can vary where you take your trip based on river levels.
Glenwood Canyon - we went last August and it was a relatively slow trip, but amazing scenery,
Poudre River - above Ft. Collins. Higher snowpacks in the northern part of the state this year may mean longer rafting season into August.
Durango - Might be iffy by August, as much of the snow has fallen in the northern part of the state this winter. Last year, when we were there in June, the river was so low, that even those in inner tubes had to pull themselves over the rocks in places, although summer rains helped a bit later in the summer.
Colorado River around Moab - Beautiful float. This would rank much higher if you were going at a cooler time. I guess the good news is that the water will keep you cool during the raft trip, but the rest of your time out of the water around Moab will be HOT.
Not rafting, but an August hiking trip up the Zion Narrows will be cool, wading up the Virgin "River" in an amazing setting.
For horse back riding, try the east side of Rocky Mountain NP above Estes Park in Moraine Park.