Grand Canyon rapids are rated on a 1-10 rating scale, versus the 1-6 scale most rivers are rated on. This is because the Grand Canyon is a high-volume river and not technical like most rivers.
Although there are some larger rapids rated higher on that scale, most days the water is quite calm with some fun, splashy, not too threatening rapids interspersed. There will be bigger rapid days as the trip passes through certain areas of the canyon such as right before and right after an interchange. If you are hiking out of an Upper Canyon trip, your biggest rapids will be the last full day on the river. If you are hiking into a Lower Canyon trip, be prepared to run some of big rapids right after you hike in!
There are roughly 160 rapids between Lees Ferry and Diamond Creek. Some of the more popular rapids on the upper portion include the House Rock, the Roaring Twenties, Unkar, Hance and Sockdologer, and some of the more well-known rapids on the lower half include Horn Creek, Granite, Hermit, Crystal, Upset and Lava.