I70 through Denver and west is probably the most scenic, but also the most traffic and the most mountain passes.
I'd take I25 North of Denver to I80. If you want, you can take I25 North a ways then take the Fort Collins (Co 14) exit to US 287; it meets up with I80 in Rawlins, Wy. I've towed through there with my travel trailer and 287 is an easy tow. Co 14 does take you through the edge of Fort Collins, but it isn't bad. It cuts off quite a few miles compared with going straight up to Cheyenne.
If you do take I25 North of Denver, I recommend you take the E470 toll road around the North side of Denver. It runs from I70 up to I25, and it's a lot easier drive than going on into Denver. It is a toll, and I think it runs around $12 if you're towing a trailer. But well worth it, in my opinion. When I go through Denver, I usually come up 287 from Texas, get on I70 at Limon, then stop at the Flying J in Denver. The Flying J is on I70 just West of the E470 toll. You can take the street the Flying J is on straight North and then catch the toll road. There's also a brand new Flying J in Limon, and one in Cheyenne and one in Rawlins. They've been pretty good places for me to stop in the past in get fuel. They have "RV" lanes for diesel which are easier to get to than the car pumps. They can be crowded sometimes, however.
At Rawlins, I usually continue on US287 to Grand Teton and Yellowstone National Parks; I understand you may not have time but if you have any way possible I'd make a stop there. You can exit south out of Jackson, Wy and head over to Idaho Falls to get back on your way.
Enjoy the trip, be safe, and good luck!