Riley Creek is the easiest 20 footer anywhere. It's only one waterfall and lands in flatwater, literally in the backyards of some nice little homes in a neighborhood. It runs year found.
Box Canyon is a few miles away and has a 20 footer, but the line is tighter, the lead-in is class 3+ and the consequences are bigger. Solid class 4 boaters might like this. It's warm water (swam in February there) and runs year round.
Not far away from there is Deep Creek, a several mile run with two 15 foot waterfalls and numerous drops rated class 4 to 5-. It's dirty water, and it runs usually only in the fall (right now!) and sometimes in the spring. This is where Boise boaters that are progressing into the class 5 realm test their creeking skills.
These runs are all close to each other a couple hours away near the town of Buhl, ID.
Also worth checking out is Succor Creek, it runs only on good years for a few spring days and is rated Class 4/5. It's near the town of Homedale, ID.
Rapid Lightning Creek is NOT a beginner run and it's way north near the town of Sandpoint, ID.
I hope this helps. Lots of photos on KayakIdaho.com
and some video on youtube if you google it.