There's two ways--you can either take I-70 to Silverthorne (through the tunnel) or use US-40 (Berthoud Pass) the whole way. Either way you will use Rabbit Ears Pass for the last stretch into Steamboat. My recollection is that Rabbit Ears lacked many of the switchbacks of Berthoud Pass. I would recommend using the tunnel. 4.5 hours from central Denver is probably reasonable. I managed that on a clear Saturday afternoon. Returning on a Sunday afternoon was probably about 5 hours or so. I would not bother with Colorado Springs unless you are saving quite a bit of money. I-25 south of Denver can be almost as risky as I-70.