If you start at the source (the Knik Glacier) the river is at first a swift class two... very cold and fun but without many of the dangers of other rivers (strainers and large rocks are very few in this area, basically there are dunes of sand and silt on both sides that don't present major obstacles. As you continue to float down it descends into the classic Alaskan "braided stream" configuration, basically small, shallow, swift moving channels amongst islands of silt.
After about three miles of this the braided stream breaks up and the river opens up into a broad, straight river with slower current and fewer "silt island" obstacles. This part is an easy class I that you could do as a family float or in an open canoe. As you float this stretch there will be beautiful scenery as Pioneer Peak rises on your left with snow capped peaks and hanging glaciers as well as dense Alaskan forest.
I've seen black bear, moose, beavers, porcupines, and many bald eagles in this area. Along this stretch the river breaks off into a series of channels, with forested islands in between them... with many nice spots to stop for a picnic. As you continue to float, eventually you will hear the sounds of the Glen Highway, and it is time to start heading for the boat take out point on the north side of the river underneath the Glen Highway Bridge.
Overall the Knik River is one of the most easily accessible floats in the Anchorage area but still provides an authentic Alaska experience.
Just a warning... the Jims Creek Trail system is most popular with the 4-wheeler crowd, which can sometimes get a bit rowdy (lots of alcohol and the occasional shooting of guns in the air)...just beware of this if you are hiking the trail to packraft and stay off the most popular trails and close to the river so you don't get run over.