Need quick advice for last-minute trip
I don't know Breckenridge, only drove there for the ice sculptures in the middle of winter... Wasn't too impressed by the look of the town, but there may be great spots and hikes I know nothing about.
I know Estes Park and the Rocky Mountain National Park very well and can highly recommend it for your trip. It's a comfortable town, just small enough to get through, end to end, in less than ten minutes, but it has everything you might need for an extended stay. And the surroundings can't be beat.
There are hikes for every fitness level. The best animals viewing is early and late in the day, and because of the orientation (mountains to the west means sun disappears early) you get great sunrise photos from many more spots than sunset pix. Getting up at 4AM is very rewarding!
For example, Sprague Lake is great for before-sunrise pictures looking east as the sky gets colored, then quickly walk around the lake to shoot westward as the first sun kisses the mountains above. The track around the lake is level, and with a bit of luck you'll see Mama Moose and more, or some elk coming to the lake early for a drink. See my recent posts here on this section of Frommers where I posted some pix from my last visit.
Here are my pictures (click).
Another pleasant and easy walk/mini-hike is around Lily lake, not within the park but off Highway 7 which runs from Estes Park south, uphill after a few miles you get to it. (Not to be confused with the Lily Mountain hike which is steep and quite strenuous.) There is parking on either side of the road, toilets, and a level ground walk around the lake. There is also a track that goes up a bit onto the rocky ledge north of the lake and comes back down near the far end of the little lake, a nice easy clamber up the path with pretty views across the lake, highly recommended. If you go up from near the carpark (turn right as you face the water, and after the first little footbridge, only 30 yards or so, step right and up you go), look for pretty lichen on rocks near the trail.
Make sure you go see the alluvial fan (read up on it, an awesome story), and drive (early - the carpark fills up!) or take the free shuttle up to Bear lake. A couple of great hikes from there - either up to the three consecutive lakes Nymph, Dream, and Emerald, or the other way, up over the hump then down towards Bierstadt Lake (about an hour) then down the steep mountain-goat zig-zag to the trailhead on Bear Lake Road and take the shuttle back up to Bear Lake if that's where you left your car.
Lots more walks and hikes, it's all good, and then there's Trail Ridge Road of course, 12,000 ft up, with awesome sunset pictures to be taken (dress warmly, it gets very cold late in the day) and stay for 30 minutes after the sun has gone down, the sky tends to go all colorful just when you think it's all over. Good spots for that: The Rock Cut (walk up above the road), or a couple of miles beyond, from the Forest Canyon lookout, and also from the last parking lot in a curve just before you get to the Alpine Visitor Center. Look at this park map.
Lately I stayed in non-descript simple motels. I used to stay at the Mountain Sage Inn but not recently since it changed hands and is now called the Maxwell Inn - it looks like they've spruced it up a bit and prices have gone up but are still reasonable. But I don't know about the separate rooms thing, and there is no pool. It's in a good location, at the park-end of the town, a short walk from the Big Horn, a hearty and friendly diner on the outskirts of the town's main drag, and from the motel it's an easy drive to one of the two park entrances.
But there are many other lodging choices, take your pick.
BTW - the park is open 24 hours, and you don't have to pay if you get in before they man (or wo-man) the toll booths at the entrance. That time seems to vary, as I was told with a wry smile when I was surprised to see a staffer still on duty long after 6PM...
Lots and lots of info is available to you - start at the park's official website - and google terms like "hiking rocky mountain national park" - there are many enthusiasts' websites to peruse.
Estes Park also has some fun things, and you can go horseback riding (very patient and friendly horses!), and maybe have afternoon tea in the Stanley Hotel of movie fame (not sure about dress rules), and strolling the main drag with its obligatory shops and more souvenir shops of course.
For picnic provisions and everything else, there is really only one store, the Safeways, open from 6AM-9PM I think, it's near the big intersection at the start of the main drag, just above where highways 34 and 36 meet - look and drive towards the Stanley and take a quick right.
From Denver you can take 36 to Boulder and keep going along the foothills through Lyons and up, still on the 36, to Estes Park, or you can take the 36 to Boulder, then take the 119 up the canyon to Nederland, take the 72 north which links up with the 7 and drops you down into Estes Park.
Via Lyons is usually a bit quicker, but not by much, and on balance I find the way via Nederland to be the nicer drive.
If you're in a hurry you might shave off a few minutes by racing up the I25 (at 75 mph limit) to the turnoff with highway 66 that takes you through the north end of Longmont to Lyons where you meet up with the 36 coming from Boulder.
In Lyons, observe the 25 mph limit or you'll get nabbed, the fines seem to be the town's major source of revenue...!
I hope somebody else knows a lot about another place for your trip. But if you decide to spend the entire time based in Estes Park, you won't run out of good things to see and do - patience and repeated visits to the same spots get rewarded by the unpredictable animals and the even more unpredictable clouds...
Have a great time!