At one point there were ice bridges across the entire river on the Grizzly run below No Name. I usually don't pay attention to this area, but I specifically remember thinking "damn, thats a lot of ice" when seeing the river just upstream of the glenwood springs tunnel. it is probably gone now, but I have not looked on my last couple trips through the canyon.
Another early season option close to denver is the golden park. at low flows you can practice eddy turns and peel outs, and some front surfs on the better waves. another option is the deckers run if the flow is good, but this may be subject to icing as well.
please do not go alone on any of these, especially for your first visit.