A bus tour of Rocky Mountain National Park takes about 10 hours round trip from Denver. Here are some to check.
http://www.grayline.com/tours/denver/rocky-mountain-national-park-5908_2/
I don't really like the Gray Line Tour itinerary. It does not actually spend much time in RMNP itself. There is no reason to stop in Winter Park. The tour enters RMNP from the western and less scenic side of RMNP. Most people prefer to enter from the eastern side. The most dramatic views are on the east side and you want to see them earlier in the day before clouds move in.
http://www.coloradowildernessridesandguides.com/sightseeing/
This tour does enter from the east side of RMNP, travels Trail Ridge Road as far as the Alpine Visitor Center and also includes RMNP Bear Lake Road and the Peak to Peak Scenic Byway.
http://www.coloradosightseer.com/4summer.php
This tour also enters from the east side of RMNP.
Do note that all of the above companies offer other tours from Denver.
Will you have a car? If so, there are several mountain areas within 2 hours drive of Denver in addition to RMNP. The Mount Evans Scenic Byway (the last 5 miles are white knuckle) starts about 30 minutes from Denver. You could drive about 90 minutes to Manitou Springs and take the Cog Railway up Pike's Peak. The Cog takes about 3 hours. Lookout Mountain is another very close option. You will see beautiful scenery and mountains if you just drive west on I-70.
Edited: 9 years ago