Wow! You get 5 weeks of vacation. You really need to give those kids a cool mountain experience and a change of scenery. It sounds like they haven't even been to one of the awesome national parks in the west.
Would it help if you let them rotate to the front seat and have your wife sit between them in the back? Have them participate and do the planning. Then perhaps they would anticipate it more.
Here's an excellent post of a #1 family vacation which might give you some ideas:
The Perfect Family Trip