The bad news is the play parks aren't very good on the Front Range. The good news is there are lots of them. I find Golden & Lyons (the latter go destroyed in the floods but I think it's being restored this year) to be the best. I wouldn't expect to see much in either of them until at least late April, but depends on the weather. You can go play around in Golden pretty damn low as there are good pools (and apparently the dredged it over the winter). Even now you can get in there, but it's more like a glamorized pool session.
Also look out for Waterton Canyon, which is short but fun and ok when over maybe ~350 or so (roughly? I think people run it as low 250 but that's low). Foxton (class III) can get some flows if they release from the tunnel, though I wouldn't count on anything early this year. Otherwise you're going out to the Ark or waiting until mid-late May for things to really start going.
- Also you can go do Confluence this low, but the features aren't very good (except @ flood levels) and the water quality is poor. I prefer Golden where I don't worry about the water.