We've tubed the entire Filter Plant run on the Poudre at 840cfs and 460cs. Both times it was a lot of fun.
We've also tubed the North St. Vrain from Apple Valley bridge to Black Bear Hole at everything from 700cfs to 170cfs. Anything below 300cfs or so will require some portaging because it is too shallow. This is a fun run but it requires some maneuvering away from the trees along the edge and during the drops (you can paddle with your arms).
We've tubed Boulder Creek at 300cfs but that wasn't much fun. The hydraulics below most of the ledge drops will flip your tube.
You'll probably have to wait until next spring for most tubing options on the Front Range.