Went down the GC with a bunch of folks last month, lots of fun!
I learned to oar an 18 foot raft and did a little whitewater kayaking as well.
We were 16 guys from Las Vegas, Yosemite, San Diego and Tahoe on a private permit.
Rule 1. Bring more beer than you can possibly imagine.
(we drank somewhere around 40 cases worth and ran out)
Rule 2. Bring more of everything else too.
Some highlights include a canyoneering descent of Deer creek canyon,( I drank beer and watched) slackline across the Colorado (not me, thanks) and did a fair bit of bouldering ( oh yeah!). We also managed a nice hand crack in the Havasu Creek area along with some easy soloing.
I cannot express how freakin' amazing the canyon is... you float past 5-600 unclimbed big walls, hike amazing slot canyons and paddle 225 miles of amazingly powerful water.
I cannot reccomend this trip enough!!!