MD10PLT

For a one day trip, the best (meaning wildest) trip in the US is the Arkansas river through the Royal George. Be sure to get a paddle boat, not an ore boat. Some outfitters tell people they will paddle but have ores also. The real fun is the smaller boat without any ores (although it's a little more dangerous). The down side of this trip is, it is waaaaay over commercialized.

For a multi-day trip, I recommend the Green River and parts of the Colorado. They have a little "bigger" water but the rapids are spread further apart. Never done the Grand Canyon. Would love to, but you have to plan this out way in advance.

mooney

I rafted the grand canyon in May about 3 years ago. May is the PERFECT time for rafting the canyon, not too ungodly hot (only averaged about 110 during the day) but the water is 48 degrees so you don't really notice it. They do fill up quick tho I'd book 8-12 months in advance. They have half canyon trips, full canyon trips etc, and various sizes of rafts. We used a company called Diamond river adventures....awesome food and service. Cool day hikes up side canyons if you're into that or just lay on the beach and vegetate. PM me if u want more...also May seems not as hard to book as later in the summer