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Easy trail and perfect weather = Mall
Beautiful scenery and pefect weather = strip club
Easy trail and beautiful scenery = you don't get to pick the weather

All of CO is darn nice, just start walking.

I would suggest leaving a stroller at home and getting a backpack for the little one. Few trails in the mountains (outside of paved ones in the parks) are going to be very easy while pushing a stroller. With one, you are confined to overcrowded trails at popular tourist spots or bike trails along roads.

Check out Estes Park and Rocky Mtn Nat'l Park. There are plenty of great lodging options (and camping), and there are tons of hiking options from "easy" to epic adventures.

I second the motion to leave the stroller at the hotel, you won't find a trail where that will be feasible other than the sidewalks that line the streets of downtown Estes. Enjoy.

I can think of two possibilities:
1. Any of the mountain towns will have river trails, either paved or a worst compacted gravel. Breckenridge comes to mind, also the trail in Glenwood Canyon. Most of these trails are currently under water, but by the 20th the snow melt should be finished.
2. For a flat road with awesome scenery, you can't do better than the Paradise Divide to Yule Pass road NW of Crested Butte. It's been closed to vehicles for ten years and runs flat at 11K for ~3 miles. Great tent camping at PD. The old road is not maintained so you'll have to deal with rock fall on the road.

Enjoy your trip.