Hi @Jsan22 - Thanks for reaching out! It's great to hear you have a trip to Colorado planned. Do you have a part of the state that you are looking to go to?
From rafting near Glenwood Springs to biking around Fruita, there are so many awesome outdoor activities to do. You may find some good information in the recent thread titled Day Hike Recommendations Near Boulder.
It all depends on your preference (distance, difficulty, scenery, etc.). My favorite is at Estes Park in the Rocky Mountain National Park. Starting at Bear Lake Trailhead you can just do the loop around the lake which is about 3/4 of a mile. You can keep going west another 1/2 mile and hike to Nymph Lake which has a couple good lookouts across the small lake. Staying on the trail another 1/2 mile your will find Dream Lake which is a long narrow lake up against a tall mountain side and is very picturesque. If you keep going another 1/2 mile you will arrive at the end of the trail to see Emerald Lake. This is like a crater lake with tall mountains all around. The best time to hike this trail is early in the morning. The goal is to get to Emerald Lake just before sunrise to see the suns red/pinkish colors on the mountains. It takes about an hour hike to Emerald Lake but that is with stopping occasionally to look at the views and catch our breath (not used to the altitude). I'd recommend looking at pictures of the hike online and seeing if that is something that interests you. It was my favorite activity for my week in Colorado last September. Hope this helps!
Where in Colorado will you be? There are so many varied experiences, including the Durango-Silverton Narrow Gauge Railroad and others; hot springs - both primitive and commercial; desert or mountain hiking; Great Sand Dunes; of course RMNP; Colorado National Monument; Dinosaur NM; Mesa Verde NP; Dinosaur tracks in Picket Wire Canyonlands (long hike to them); and whitewater rafting, caves and waterfalls, to name a few things. If you could give me an idea of location and interests I could narrow it down for you. What I wouldn't plan for is climbing a 14er in May unless you have extensive mountaineering experience.
I've lived in Colorado since 1985 and have explored the state quite a bit. I'd be happy to help guide you so you might come to love it as much as I do.
Around Denver I'd hike in Mt. Falcon Park, Castlewood Canyon SP, Golden Gate Canyon SP. Rosalie Trail is quite nice, too. The higher you go the more chance of running into snow.
For sightseeing types of things, go to Pearl Street Mall in Boulder. The Denver Art Museum is very good. Drive up Pikes Peak. Drive to Rocky Mtn NP on Hwy 36, but return on 7/72/119/6.
Colorado is the best! Am I biased because I am from Colorado and I've spent most of my life here? Probably!
The outdoor opportunities here are endless--literally, endless. I'm also happy to help make recommendations depending on the length of your trip, expertise of you and your party, access to a car, and desired views (snow? mountains? high desert?)
The weather in May will definitely affect your outdoor activity choices. Last year, we had a large late-May snow, which is not always typical, but can definitely happen. I'd recommend having lots of different options for an outdoor activity if weather should decide to intervene.
Let us know how we can help, and definitely send us a post-trip report!!