At Caras Park in downtown Missoula, there's a standing wave in the river that people surf on.
In the residential neighborhoods of Missoula, most of the intersections are unprotected--no stop or yield signs. Word is that when college students arrive in the Fall, they're unaccustomed to unprotected intersections and mayhem ensues.
Have you ridden the Route of the Hiawatha rails-to-trails trail? It's near Wallace, Idaho, which you said you'd been to.
We had our own bikes, but you can rent them. And you can get a bus from the bottom to the top. We rode both ways, downhill first and then a slight constant grind back up.
You probably have to make all kinds of reservations these days. We did it on the last day of the season about 10 years ago and just showed up and it was rainy and kind of cold but I still loved it. We left the motorhome in Haugan and drove over, and it turns out we'd crossed into a different time zone without realizing it, which doesn't matter unless you're wanting to take one of the buses.
I also second the recommendation of Little Bighorn. It's the only place I've ever been to that I can actually see what they show on the map--where everybody was at certain points in time, etc. It's really well done.