First, I am the owner of Pucon Kayak Hostel and am understandably biased to Chile. But I'll be honest with general travel tips and river specs.
My experiences in Central and South America are Ecuador, Costa Rica, Argentina, DR, and Chile. And each of these can offer all of the adventures you described.
You mentioned bus stations. My first advice is to be your most protective at the bus stations and on the buses. All of these destinations use the "distract and snatch" method of petty theft. I've had friends lose their camera while they snoozed on an Ecuador bus. A girlfriend have her window broken in Costa Rica. And I personally have chased a bandit from my bedroom at 5 am on Christmas morning in Chile. If I am in a bus station I tie my bags together, or to my body, or to a table. Tying bags to things is a preventative of the snatch and run. I also suggest the Clint Eastwood method. How would Clint protect himself in those classic cowboy movies? Sit in the back of the bar with your back to the wall looking like a badass. By making your bags hard to get to and hard to escape you've already helped yourself.
I think what you described for busy water boating sounds like Ecuador would be a hit. Chile has lots of waterfalls but there are even more runs without cascades too. Pucon Kayak Hostel is based right at the takeout for the Upper Trancura and putin for the Lower Trancura. Other runs in the area without cascades would be Puesco, Maichin, Sand Pedro, Middle Fuy below the 50 footer canyon, Lower Fuy. To the north would be the Maipo and Nuble rivers. And to the south the Futaleufu attracts lots of annual kayakers.
PKH offers everything from show-up and do-it-yourself by renting a boat and shuttle to all-inclusive guided trips to destinations like the Futa and Teacups. We'd be enthusiastic to host you and happy to offer free logistics advice.
What other travel tips do you need help with?
Owner, Pucon Kayak Hostel