Not sure exactly what zone the area near Missoula would be but we have a Sugar Maple in our yard that we love. We are considered zone 5. This link suggests that you could be anywhere from zone 3-5, but the Sugar Maple is good in zone 4.

Here is a link that might be useful: USDA Plant Hardiness Zones - Missoula MT.

My two favorite ornamental trees are 'Autumn Brilliance' serviceberry and 'Princess Kay' plum. They flower beautifully, have nice form, and get good fall color (well, the plum is 'good' - the serviceberry is 'phenomenal!'). The sources I checked show both as a zone 3/4.
Happy hunting!

Thanks for the ideas!

Japanese tree lilacs can be hardy to zone 3a. They don't get overly huge, they are very nice looking and smell amazing.