Milk River - The Milk River is an excellent early season paddle with both scenery, rattlesnakes and whitewater! It is typically done in 2+ days, however portions of it can be run as a day trip easily. Expect plenty of small rapids with a few more challenging class II+ rapids. At high water you can expect some larger waves and easy class III rapids.
A night at at “Poverty Rock” id highly recommended, a scenic and fun campsite partway through the run that has been arranged between local paddling groups and a farmer. If you have never visited the Badlands or Writing on Stone Provincial Park this is a great way to really appreciate the geography and history of Alberta. It is well worth planning to spend some time at Writing on Stone exploring the hoodoos and pictographs.
For more information you can see Mark's Guide for Alberta Paddlers, Mark Lund has exhaustive information on the run in his great book.