OP,
Others have given you some great suggestions! The ages of your kids will determine more how to apply the suggestions given than any other factor. What pre- and early- school kids enjoy will differ from middle and high school age kids.
In one of your comments, you indicated that you were looking to NOT go north of I-80, as you indicated that there may be another western trip in the future.
My suggestion, depending on the ages of your kids -- start planning 2 or 3 different trips now, based on these suggestions. If your kids are really young, 3 trips west before they graduate high school could be doable. If they are teens, 2 trips could still be doable.
On one trip, focus on California, if that's a must see type trip. Certainly, others have given great suggestions for the Golden State.
For another trip, focus on the 4 Corners States. There is such diversity in these 4 states and lots of good suggestions have been made previously.
For a 3rd trip, focus on that "north of I-80" trip...Yellowstone/Tetons, the PNW, Glacier, etc, with a possible excursion in to Canada depending on your time element.
If your kids are very young, you'll need to allow for less time driving, more time camping. Been there/done that/won the t-shirt! Back in the day, when my kids were young, we sacrificed expediency to quality of time at locales and kept our itineraries more focused. As the kids got older, we could broaden that. You may want to consider doing something similar....again, depending on the ages of your kiddos.