If you are a first timer and want an overnight that you can get a permit for do Ruby Horsethief. It will be more of a drive from Winter Park, but is not particularly technical and is very suited to a birthday celebration with the guys. I just checked recreation.gov and most sites are still available for the 23rd since it is a Thursday and a couple for the 24th. Listen to DoStep's advice, anything else in CO would be out of your league and hiring an outfitter would make it a lot more pleasant/safe/stress-free.
For boat rentals start by checking the mountain buzz partners, there a a bunch of rental services that support this forum.
"Ski bums need something to do in the summer, and river rats need something to do in the winter." -Old Colorado mountain bum adage