I've been back/forth between WI and CO many times over the years, the usual route being I-80 just for expediency. However, if you're looking for good routes for getting off the interstate there are a variety of US highways that will do the trick. You won't, however, have any mountains until you actually get to Colorado.
A favourite and recommended route from/to points east would be US 36 across Missouri and Kansas, coming in to Colorado and hooking up with I-70 east of the Denver Metro area. Another route could be US 20 across Iowa and Nebraska, and then US 385 south to I-80 and on to I-76 to take west to US 34, which takes you to Loveland CO and the gateway to RMNP. US 34 also runs across southern Iowa and southern Nebraska, so depending on where you are in Wisconsin, that could also be a good scenic route. Frankly, anything off the interstate will be more scenic!
You don't indicate your starting point in Wisconsin or when you plan to do this. Depending on when (before Memorial Day 2017), there's a detour getting from Loveland to RMNP due to construction. That becomes a moot point after Memorial Day 2017. Also depending on the "when", weather could be a factor.
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