Living in Colorado, I think I can help. Colorado has a LOT of wonderful places to visit, for landscapes, wildlife, and more.
The most well known would be Rocky Mountain National Park, just north of the city of Estes Park. From there, you have everything your looking for...lots of wildlife, lots of birds, lots of nature, tons of wild flowers (at least now...they are blooming and will be done soon), and some truly amazing landscapes. You will want to take Trail Ridge Road up to the continental divide...absolutely spectacular! You should also visit Bear Lake, from which a whole host of trail heads spin off. Bear Lake itself is beautiful, but you can take trails up to Dream and Emerald Lakes below Hallette and Flattop Peaks. You can also head off in another direction, and climb to the basins just below Longs Peak (a 14,000+ foot mountain) at he head of the divide. There are also dozens more trails.
You can take longer hikes up to other truly amazing vistas in other areas of the park. Most of those trails are longer than the ones around Bear Lake, so you'll have to prepare. If you take Trail Ridge Road up to the divide, you can continue along it and go over the divide to the western side. More beautiful vistas there. There is also Lake Granby, one of the largest lakes in Colorado (second largest), but has some amazing water vistas with the divide behind it. Just to the south of RMNP is the Indian Peaks region. I actually spent the last few years of my teens growing up in this area. It's truly amazing, another huge range of incredible peaks stretching down parallel to Peak to Peak highway. There are lots of little county roads that take you west towards the mountains, and dozens of trails to follow. I recommend hitting up Brainard Lake, and take the trail to Long Lake. Beautiful landscape up there, wonderful place for wildflowers, beautiful creeks and rivers, and some more unique birds (such as the Gray Jay...truly beautiful birds!) Later in the year, it's also a wonderful spot to get macro photos of mushrooms, of which there is a pretty wide variety.
Another wonderful place is the Mt. Evans Wildlife area, and Mt. Evans itself. This is one of a few mountains you can drive right up to the peak of. The vistas from up there are amazing...you can see for miles in every direction, and it's stunning at sunset. Last time I was up there, I was a noob of a photographer, so I don't really have any nice photography to share. But I do remember the alpenglow...dozens of peaks all glowing at the same time right at sunset...just absolutely amazing. There are a few mountain ridge trails that you can travel to get to some neighboring peaks around Evans, and there are a number of lakes dotted around.
One of the more iconic spots in Colorado is the Maroon Bells. These are highly recognizable because of John Fielder's work, and he has photos of the Bells in every season. The Bells themselves take a while to get to...it's a good long drive from pretty much any of the major staging towns (mainly Aspen, CO IIRC). If you are willing to hike, the high plateaus above and behind the Maroons are also just phenomenally beautiful. A large format film photographer, Jack Brauer, has some stunning photos from the high altitude areas behind the bells.
The continental divide stretches through Colorado's entire height. You can find 14ers all along the central ridge, including Mt. Elbert (tallest in Colorado), Mt. Massive, La Plata, Mts. Oxford, Harvard, Columbia and Yale, as well as a host of others.
Just to the west of this lower region of the divide are Aspen, Glenwood Springs, etc. These towns are beautiful, and can be staging grounds for visiting the other mountainous regions nearby. Glenwood Springs is home to some hotspring-fed pools. If you can hit them on a weekday, it's better...as there are fewer people...but the pools can become really packed. There are also some caves that you can visit near Glenwood as well, if your interested in that kind of thing (there are actually a number of interesting cave systems around Colorado, something to keep an eye out for.)
There are a couple other amazing regions as well. Colorado has a small desert, Great Sand Dune National Park. This is home to the tallest sand dunes in North America, nestled just behind (to the west) of Pikes Peak. I haven't been down there since the 1990's before I moved to Colorado, but a photographer friend of mine, author of a photobook for the park, has some amazing photos of the area.
Finally, if you have the time, the south western quadrant of Colorado has some truly stunning landscapes. Again, Jack Brauer has truly amazing photos of this region, as it's where he lives. South western Colorado is home to the Uncomphagre and San Juan mountain regions. Littered with aspens and many high open meadows, it's just stunning. I can't describe it...just check out Brauer's work. It's a long ways from the populated "center" of Colorado, but that region is definitely the "heart" of Colorado. It's out of the way, there aren't many towns down there, certainly no real big cities...but if you have the chance, it's worth a visit.
There are plenty of other interesting places to visit. One of my favorite places near the urban and suburban sprawl of the greater metro areas around Denver is Roxborough State Park, which is full of giant red slabs of earth that jutt up from the earth. Another similar, but also distinctly different, place is Garden of the Gods near Colorado Springs. Small places, but worth a visit if you have the chance. Well, that is all barely scratching the surface. There are thousands of incredibly picturesque places to visit in Colorado...impossible to list them all. I think I've named the major regions for you, though, which should get you started.