In a raft, I assume?
Best, closest thing is probably the Upper Colorado below Pumphouse. The lower stretches of the Arkansas around and below Salida offer good options too. If you guys are going to be using duckies or a small raft you might look into the South Fork South Platte between Deckers and the confluence with the North Fork. I am not familiar with the South Platte around and through Denver, but I have heard there are a few options.
Hope that helps.
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