Radium Hot Springs is a primitive pool (meaning no bathhouse, structures or amenities) near Denver. It is the closest spring(primitive) that I know of, coming in at about 2.5 hours drive from Denver. The spring is a small one along the Colorado River about a mile hike from the parking area. The spring is on the warmer side as opposed to hot, however none the less soothing. A passenger train passes through occasionally, and there are lots of rafting and canoes coming by in the warmer months. Accessible via most vehicles, however I have always been in a 4x4/all wheel, but only in the winter is this necessary.
I have been to this spring numerous times, and always the drive varies. For example in the winter on the weekends you're pretty much doomed by traffic from the slopes. But then again I went for a late night soak once in December for the full moon and it was lovely, no traffic due to the night hour. In the winter leaving Denver between 7am-10am I always get stuck In traffic, so plan ahead and leave according to your schedule and patience for traffic.
Take US-6 towards Lakewood then exit to merge onto I-70 W towards Grand Junction. Then take exit 205 onto CO-9 N towards Silverthorne, US-6 E. Keep right onto CO-9 N towards Steamboat. You will be on CO-9 N for about 30ish miles, passing the Green Mountain Reservoir. You will go through 2 tunnels, after the second tunnel (about a mile) you will take a left on Trough Road (my phone looses service before this point). You will be on Trough Road for 14 miles, it is a dirt and gravel road. Soon after you will see a BLM sign, turn right and continue for a mile, turn left at the campground called Mugrage. Hike up the inclined rocky hill across the road from the parking area and continue on the path towards the river ahead until you reach the cliff edge. The path will be slightly pushing right while approaching the river cliff. There are a few ways to descend the cliff, do which ever path is comfortable for you, but you know you are there when you begin to see the the rocky paths down the cliff where you will see the pool below. If in doubt find a safe way down the cliff and keep walking along with the river on your left, you will eventually run into the pool. There is also circle of rocks at the top of one of the ways down from a past campfire.
What to expect~
A warm shallow pool in the Colorado wilderness large enough for 6 people or so. There maybe some snow depending on the time of year, and other soakers are common. Cliff diving, canoes and rafting are also a common activity you may see here. A train comes by often as well. In the winter months the river will be frozen over.
What to bring~
Towel, robe, water, snacks, speaker if you want music, trash bag for trash (pack in pack out), wet bag for swim suit, flip flops for walking around with wet feet, camera, whatever else you want.
DO NOT LEAVE TRASH, EVER. I also do not bring glass just because its risky. Also think twice before bringing a pet. Personally I have never brought my dogs to a hot spring for I do not feel like dealing with it during my serene soak. Plus how safe is mineral water for dogs, I certainly don't know or wish to risk it. HOWEVER, I have been to Radium where someone had brought a dog, it was sweet, polite, and calmly sitting in the water with us totally chill and didn't stir up the vibe. Then again, the last time I was at Radium someone had 2 rowdy dogs there off leash barking constantly and one ended up biting my friend in the face. So please be cautious and make sure the dogs are safe, and comfortable in this type of environment.
***Cell signal is about 45 minutes from Radium so first aid kits are a good idea to have.***