Sky is the limit given where you are.
Upper main Blackfoot (as you posted elsewhere) is a good one. The lower main Blackfoot is pretty mellow in a packraft once the water drops a bit, but that usually coincides with when it gets crowded.
The entire Bitterroot is gorgeous and easy, but again can be crowded. If you put in after the drift boats hit the water late morning you’ll see fewer people.
Rock Creek has a rep as being tough in a big raft due to brush, but its pretty easy in a packraft.
Ditto for the upper Swan River, which has surprisingly little evidence of civilization. Dirt only bike shuttles possible.
Clearwater canoe trail is the definition of mellow and wouldn’t be too slow in a bigger packraft.
South Fork is mandatory for sure. Go when it’s between 2-3000 cfs.
North Fork of the Flathead is also excellent. Three real rapids of note on the whole thing, easily portaged, and again pretty mellow at summer levels. Bike shuttle on the Inside NF road is one of the better dirt two track rides in the state.
South Fork of the Sun below the South/West confluence is a great fairly mellow float, if you portage the little gorge right before the start of the reservoir.
In Glacier NP Camas Creek between Roger Lake and the Inside NF road is class 1 and dead gorgeous with tons of wildlife. Float this at high water if possible.
I never floated Monture below the campground. Gradient seems kind, I’d watch out for brush and possibly fences.