If you take I90, you'll be following the Clark Fork river going west out of Missoula. Leave the interstate at Wye and follow the river on HWY 200, all the way to Lake Pend Oreille. Worth the drive, even if you never wet a boat. Last I checked, there are shuttle services, and much of the river is canoeable. The lake itself is worth checking out as well. Lots of camping opportunity on the route.
From Sandpoint at the north end of the lake, you can cut back down to I90 via HWY 93.
Or you could diverge a short distance farther north from Standpoint and paddle "The Thoroughfare" out of Beaver Creek campground into Upper Priest Lake for a day trip or an overnighter. (Highly recommended)
OTOH, if you wish to stay on I90 and skip Clark Fork - Pend Oreille, you may want to stop for a day and ride the Hiawatha Trail. It's a bicycle trail that runs on the abandoned Milwaukee Road railbed east of Wallace and runs through several high mountain tunnels and over several high trestles. It's easy pedaling with fantastic views, and there's a shuttle system that runs daily. Bring a headlamp and a spare light. Bring your own bike if you can. Rentals are available, but you need to reserve or get there early. It would be a shame to drive right by and not do that.
The trail starts just off of I90. Registration and rental are at the ski resort office (also right off the interstate, and a short drive to the trail). Riders are allowed to RV or car camp in the parking lot, which is surprisingly quiet.