It is really hard to see Denver and Rocky Mountain National Park in one day. Rocky Mountain National Park alone requires full day even you drive only without hiking or camping.

I think you are talking about Estes Park which is the city next to east entrance of Rocky Mountain National Park. Estes Park is 60+ miles from Denver and it will take 1.5 hrs. So it will be 3 hrs two way and you will need at least 1 hr in Estes Park just to see the place. Now if you don't have enough time I won't suggest you to buy entrance pass that will cost you $20 and enter the Rocky Mountain National Park.

If you have 3 more hours then you can enter the park, go to Bear Lake and drive along Trail Ridge Road. But you need a strong heart to drive that road. If you are not experienced in driving mountains roads then don't try it. You will be driving along very narrow and curve road without any support on downhill side and the road goes above the tree line where you will see steep bare cliffs of Rocky Mountains.

BTW every elevated topography towards the west of Denver is part of Rocky Mountains, except few hundreds feet of front range that goes from north to south parallel to Rocky Mountains in the east.