Folks, I had a 12 day trip planned from Houston to Rocky Mountain to Dinosaur National Monument to Yellowstone to Arches to Canyonlands to Carlsbad to Houston. I want to add 4 more days to the trip and I feel like I should spend that time in Utah or Colorado area. But I am all ears if you have other recommendations within WY, UT and CO.
If you had 4 extra days to spend in WY or UT or CO, where would you be? Splitting time between places is fine with me. I had scheduled only a day (and night) at Rocky Mountain and that's what made me think I should add days and spend more time.
I've been to all of those - happy to help:
Canyonlands is a mini "Grand Canyon" - beautiful and a lot of hiking opportunities (if that is your interest) - I did a day there and barely scratched the surface, so 2 or 3 is probably the desired. It's only about 20 miles (from memory) from Arches, so you can basecamp somewhere and do both. Portal Resort in Moab would be a high-recommend from me, and it's like 3-4 miles from the Arches entrance.
Arches - mostly a car-tour. You can get off and hike, but they are mostly short walks to see the arches up closer. A lot of it is geologically and environmentally sensitive, so there is only so much you can do. 1 day is easy to see most of the sights, 2 days would be ideal but any more than that may get boring. They are awesome at first, like any interesting view, but sort of like looking at the bison at Custer Park in South Dakota - the first one is fantastic, but after #435 on the same road, you sort of start cruising by.
Dinosaur National Monument - it's a day trip as well. The Vernal area is "meh" - I would just stay in the park if you can. The dinosaur area is very "managed" and it is basically a rock wall inside of a museum type of building. The longer-items would be hiking or touring the park in vehicle. My wife and I liked the petroglyphs that look like aliens.
The drive out to Vernal can be a variable - lots of truck-based oil-shipping in that area, so probably add 20 or 30% to your assumed drive-time.
My recommendation would be to add the 4 days to see Zion (1 or 2 days) and Bryce Canyon (day trip from Zion base camp). My wife and I like WillowWind in Hurricane, UT for seeing Zion - you won't get anything near the park during high season if you haven't already reserved it, but WillowWind should be doable and it is a great location / great park. You can do a daytrip to Bryce from there if you have a runabout vehicle (we tow a pickup behind our motorhome - not sure what your situation is). Zion would not be friendly for arriving in a motorhome.
At Zion - as a trick - it is only viewable by the trams (free) - it's a hop on / hop-off road-train thing. The visitor center parking lot fills more or less instantly, but we always find (easy) parking about a ¼ mile from the Zion gate in a private parking lot behind a coffee shop that is on the main drag through Springdale - I think it's called "Canyon Coffee" or something like that and they rent the parking spots with a kiosk for $20 all day. You can walk in from there (easily).
Carlsbad - not really an opinion, I assume you mean the cave stuff. I have been through but haven't stopped, if it's like the Craters of the Moon in Idaho, it's a 1-day thing.
Colorado - from Moab it's a short drive to Colorado but I think it would be the Grand Junction area, Vail, etc. I guess the question is when are you going? I live in the mountain west, we are still getting rain (and some snow) (today).