Tony,
You have so many choices.
One would be a little longer by driving down the #101 along the Pacific Ocean which is gorgeous, (watch out late in the day for logging trucks, especially at the curves), they don't stop for anything or no one. When you get near San Fran. work your way over to the I-5 and
take a left at I-80 after you cross the Golden Gate Bridge to Salt Lake, take the I-15 South to Provo and then take the secondary road Route #6 to I-70 through the area of Aspen (side trip, but not long), Vail, and Breckenridge into Denver and continue on I-70 to I-135 and hang a right. This interstate turns into I-35 that will take you into Dallas.
If it was me taking this trip, this is the way I would go for the most scenic route.
You have so many choices, but no matter which way you go, stop at scenic areas in Colorado and bring peanuts/crackers for the ground squirrels that will come up on your lap and eat from your hand, also, stand on high ground and hold a peanut or cracker high in the air and a hawk will swoop down and take it out of your fingers and you won't even know he was there, except the tid bit is gone. Have someone ready with a camera.
You might consider the grand Canyon on the way back off I-40 which would be a bit different route and then work your way to Hoover Dam just below Vegas and you will see the new arched bridge they are working on just about Five hundred feet West of the Hoover Dam which is being built into the bed rock, then their is Vegas, Coins at the ready here. Then back up to Salt Lake and you can take a shorter and quicker route back home.
Enjoy the ride and this wonderful car.
Hey all I'm going to be driving from Seattle to Dallas in about 2 weeks and was wondering what's the best route to take as I would like to get some sight seeing in and experience a nice drive with my baby...