Vegas to Philadelphia...need help

So a friend of mine and I are going to be traveling from Vegas to Philadelphia this summer. It's the cliche finally graduating from college one last mother of all road trips type thing. We will be flying into Vegas and renting a car in order to save our cars on the mileage. This is not our first road trip, but will be the longest by far that we have taken. I've done quite a bit of research, but was still hoping for some suggestions and help.....

We are thinking as of now, this will be our itinerary...

Fly into Vegas early and spend the day there.
Wake up early the next morning and one of the major attractions for me on this trip is Zion National Park. The problem is the Grand Canyon is also a major attraction for us. So we were thinking travel through Zion on our way to Grand Canyon and then spend the night in Grand Canyon City. I am a little shakey on this part b/c I've never been in that area so I am not sure what to expect. Any suggestions on altertnate routes or ideas on other attractions in and around Grand Canyon? Possibly evening attractions i.e. resturants, places to hang out etc...
After leaving Grand Canyon we were debating whether it was pratical to drive all the way to Boulder Co, in one day, or break it up into a two day drive to kind of make it more enjoyable. The four corners is another attraction of ours, but I am not sure of the area and what would be a good place of middle ground to stay. Obviously we would like to cover some ground that day, so some where in CO. would be ideal, in the southwest corner I was initially thinking. Suggestions on cool towns in that area?
Then after arriving in Boulder, I am relatively familiar what the attractions there and should be comfortable finding my way around. Althought after speaking with a friend who has lived in CO for a few months, mentioned that US - 25 is a cool road to travel down b/c it goes along the rockies and many other parks and attractions. Can anyone confirm this? From Boulder we decided to make this the long part of our trip. Boulder to Oklahoma City. I have an vague idea of what this city was like, but to my surprise, the city seems much more livlier than expected. I am kind of lost thou as to what are some good attractions in the city. I checked the visitors guide to all the cities we have pin pointed, but found very little on their site. Any suggetions again are greatly appreciated.
From Oklahoma City we are again undecided were to go next. We want to find a city in Texas to stop at that is on the way to New Orleans, but again being unfamiliar with this area, I do not have a clue where to go. We were talking about Marshall, or Longview, both seem nice, but no real attraction draw for us. Dallas is an option, but wondering if anyone had any suggestions on good places to stay in Eastern Texas.
After Texas, I have a pretty good idea of where to go and what to see. I have friends in most of the other cities stops, so I guess its more the Western part of our trip I am a bit stuck on. I know i've probably asked alot of very general questions, but I hope and appreciate and help given. Thank you very much again for any responses.

The route from Zion to Grand Canyon Village is a beautiful drive and shouldn't pose any problems for you. Don't sweat it. Just sit back and enjoy the scenery.

I would definitely add another day between Grand Canyon and Boulder. First, it's about 700 miles. Yeah, you can drive it in a day but you won't have time to see the amazing scenery that you'll just be zipping by. That would be a shame.

If you leave Grand Canyon by going up to Tuba City and taking 160, this will take you directly past 4 Corners. 4 Corners is a fun stop. I think everyone should be in 4 states at the same time at least once in their lives. Great photo opp. But I've never driven all of 160 so I can't comment on that route (I came to 4 Corners from another direction). I'm sure it's fine but, if not, I'm sure someone with experience on it will weigh in.

Personally, if it was me, I'd consider staying on 160 after 4 Corners and spending the night in Durango. This is a fanstic, mountain town. Beautiful drive to get there and the town itself is in a magical setting. I think you'll find plenty of fun things to do there. Unfortunately, I didn't have time to spend much time there so I can't comment specifically but it looked like a town with lots of good dinner/nightlife options, in addition to outdoor activities and historical stuff. Grand Canyon Village to Durango is still a bit over 400 miles so it's still a long day's drive. But not so long that it's not do-able.

From Durango, you can either head north on the Million Dollar Highway (550). I've not done this but I have heard many times on these boards how breath-taking it is. Or you could continue on 160 until it meets with 285, taking that north through the Rockies, then veering over to Boulder on 24/285. I would do that vs. going on I-25. It's an amazing drive.

Hopefully this will get you started.