Hmm, I'd have to say one of my absolute favorite cruise excursions was one that was arranged independatly out of Manaus, Brazil. 8 of us took a speedboat up the Rio Negro a ways to a spot where we could get in the river and feed and swim with the pink dolphins. Then we spent some time at a sandy beach on the river, then headed to a native's house for lunch. Finally we visited a native tribe at a reservation set up for them to practice their traditions and went through a welcoming ceremony. Sunburnt pretty badly that day, but it's a trip I'm always going to remember.