I just went to RMNP two weeks ago. You'll want to get a car pass. One tip is that 40% of the tickets are held back for release at 7pm the night before. I was able to easily get a pass for my arrival window with zero issues the night before. I recommend getting the pass with the Bear Lake Road add-on. If you go to the end of the road, there's a big parking lot and some pretty easy but enjoyable hikes...one goes by 3 lakes and then the loop around Bear Lake. There was still a lot of snow and ice on the ground two weeks ago but I'd bet it's gone now. I had brought my hiking boots without GoreTex so my feet wouldn't get hot but it would've been better wearing them since I had to pick my path to avoid getting my boots wet. If you backtrack down the road, there the Glacier Gorge Trail and Alberta Falls. You can fill up the day with those trails. Back in Estes Park, there's a run down burger joint called something like Burgers-Gyros. It was a damn fine burger.
If I was mostly staying in Denver, I'd just stay downtown and have that extra 45 minutes drive to RMNP.