I don't get it. I live in Wyoming and have 100,000 miles of touring experience - most of it in the American and Canadian West. The West isn't Yorkshire. To get from Boise to Boulder you have to cross the Continental Divide. By definition - that implies some mountains. Yes, there are lower places - but the most prominent is already occupied by Interstate 80 - and although legal to ride - certainly no pleasure. I-80 is a nonstop stream of traffic - a third of which are 18-wheeler trucks. (Lorries in British speak)
I am guessing that one reason you have not yet received any replies is that you are asking the impossible. Either you will have to ride long stretches on I-80 - or will have to chose smaller routes which involve more climbing. I-84 is similar, but with less traffic. There is not a continuous service road (Old US 30) along I-84 or I-86 and when there is, it can come to a dead-end with a high fence separating you from the interstate. Also - you don't say when you are planning to do this trip. Southern Idaho can be brutally hot in mid-summer with little to no shade for miles. Doable, but no cakewalk.
There are a number of pleasant possibilities that avoid most interstate sections - but do involve some moderate climbing. Let me know more about what your plans are and I will offer suggestions.