I've driven my '05 Prius over Logan Pass in Glacier Nat. Park (elev. 6646 ft.) with no problems. IMO it should not be a problem to drive to Sunrise in Mt. Rainier Nat. Park in your Prius. Sometime this summer I plan to drive up in my Prius to Paradise in the same park.
No sweat, and, you will get fantastic mpg readouts on the way back. Bump up your tire pressures to 44 psi front and 42 rear and though the ride wil be more harsh your mileage will go up and the handling will improve.
Anybody from the Denver area wish to chime in with their experiences in a Prius going through Eisenhower Pass on I-70 at over 11,000 ft?
My $0.02