For a raft, the best options would be:
-Kicking Horse River (1.5 hours west in Golden BC) Class IV commercially rafted river. Cold, continuous, big water. Be careful of wood.
-Kootenay River (1.5 hours south) class II-III multiday with 2-4 day options
There is some serious steep creeking near Banff, but not really great for a raft (Redearth Cr., Yoho River, Johnson Canyon, and the Pipestone River)
The Bow River provides a mostly class I scenic float and runs right through Banff (with a V+ waterfall).
The Kananaskis is 45 minutes east of Banff and is a short II-III artificial slalom and playboat course, commercially rafted, but more fun for messing around in kayaks.
There are other options south in Fernie, and west in Revelstoke, and north in Jasper.