Colorado is one of the most incredibly diverse states i've ever been to, and i had the pleasure of living there for a few years during the pandemic, so I made my way around a lot! Depending on what you're looking for, there are some great options to choose from!
Before You Go
US Annual Park Pass: If you live in Colorado or visit national parks more than ~3 times per year, then you should 100% have an annual park pass. It pays for itself after about 3-4 visits and is accepted as entry for many of the state parks and camping sites in Colorado. This pass also includes parking, so overall is worth it. You can buy it online or at REI.
Check the weather! Weather can change dramatically based on where you are and the time of day in Colorado. Thunderstorms are extremely dangerous due to high elevation, especially if you're above the tree line since you'll be the tallest "object" for lightning to strike. Avoid hiking in the afternoon because thunderstorms are pretty much a given during the summer then. Go early and finish early!
Take Advil or Ibuprofen: Altitude sickness is no joke and can truly ruin your experience. The highest point on the park road goes above 12,000ft in elevation, so it's likely to feel a little sick from that. If you're hiking a difficult trail you'll likely feel that elevation even more intensely.
Choose your Trails ahead of time: Trails are endless and it's 100% recommended to use Alltrails and select the best route for your trip goals, this will also help you figure out which trailheads to navigate to, since some sections of the parks can be hours apart.
Rocky Mountain National Park
For all summer you need to have a entry permit reservation through recreation.gov for a specific timeslot to enter the park between the hours of 9AM-2PM. You don't need one for before or after these times though. There's not much convincing needed for you to visit here. Many of the most popular hikes start at the Bear Lake trailhead, so if you want to do anything remotely popular you'll have to arrive early.
Where to Stay
I usually make the day trip from Denver, but if you're exploring for several days, you should probably camp. If you have a dog, you'll want to get a pet-friendly hotel since you can't bring them on mostly all trails in the national park. If you're a fan of the movie The Shining, stay at the famous Stanley Hotel (or at least take a tour of it!). The Estes Park Resort is also a popular choice.
If you are short on time and don't mind long hikes, then I highly recommend you do the 11-12 mile hike where you can hit Emerald Lake, Sky Pond, and Lake Haiyaha (Lake Haiyaha changed in hue due to a mudslide which is what has created that stunning blue color in recent years). I've gone before and done just Emerald lake, but after returning and doing the full loop I wouldn't do any other way, that way you only have to spend one day visiting the park. Most of the pictures below are found on the same hike! We went in September and the temperatures were perfect. If you don't make it to the Bear Lake trailhead, then plan to do the entire loop drive around the park, go up to the highest point and take in the views from the visitor center up there! You can also find less popular hikes anywhere along the drive, they're all going to be gorgeous.
Black Canyon of the Gunnison National Park
If you're looking for views that are par with those of the Grand Canyon and you're hoping to do some stargazing then I suggest driving down past Montrose and visiting here. It's a registered Dark Sky park, meaning it's far enough from cities to prevent light pollution from disrupting your night sky views. The canyon itself is super underated and not heavily trafficked due to how far away it is from Denver. The park was virtually empty when we went in August, no line to get in. You can drive along the route and stop by each viewing point; I wouldn't recommend this park for hiking though, just for views and camping. It's extremely hot here, so if you want to hike, just try and go during cooler seasons.
Where to Stay
Check out the moon phases and try to reserve a campsite or stay at a hotel in Montrose (about ~10 miles away) nearby during a full moon! I've camped several times at the official campsite and it was so cool exploring the rim trail at night and capturing the milkyway.
Great Sand Dunes National Park
This is bit of a drive south, but it is SO worth it! These sand dunes just appear over the horizon as you're driving down and it's absolutely stunning. I dreamed of going to this park for awhile and it did not disappoint. It's such a unique landscape that you can't get in many places.
Where to Stay
The line can be long to get in, so arrive early if you can. Camping is also really popular here, I've stayed at the Pinon Flats Campground before and it was perfectly situated right next to the dunes; this is a must if you're doing night photography. You can also stay at the pet-friendly Great Sand Dunes Lodge if you need additional accommodations.
If you're bringing your pup, then make sure you have goggles and shoes for them since the wind is very strong and the sand is extremely hot on sunny days. (pro-tip: wear glasses if it's windy! I had to go to the eye doctor to remove sand particles that got stuck in my eye) I went on an overcast day and lucked out with cooler sand temperatures. If you go end of May, you can catch the creek flowing which is super refreshing.
As for hikes, I honestly just show up and follow the crowd to the top of the mountain. You can get backpacking permits and have more privacy on the dunes as well. There's so much space though, feel free to bring your long-line leash, just stay away from bothering others who are enjoying the park! Zapata falls is also nearby, so you should check that out too!