For RMNP, you'll want to drive up on Fall River Road, then come back onto Trail Ridge Road. Fall River Road is one way only (up). The pretty way to Denver from Estes Park is to take CO 7 to CO 72 to CO 119, which is the Peak to Peak Scenic Highway
, to I-70, but the fast way is US36 through Boulder.
There is a middle ground -- CO 7 to CO 72 to Nederland, then take CO 119 into Boulder, which lets you stop at Boulder Falls, which are rather impressive.