I think the tours are just random each day, you get lower level tours show up even when you can easily do shark hunting or giant dragonfly tours. For instance today I had the giant dragonfly tour and the butterfly discovery tour on my list.
Elite Scavenger. One could make the argument for Tuna Kahuna and Giant Dragonfly but I sure you can 2 Elite Scavengers in the time it takes to complete one Giant Dragonfly since those times are fixed. 20<32, obviously.