I'm not sure if this is unique, but my Gem collecter went to China and came home with some books that had appeared in her inventory. Plus she found some bags of cash, some coins, artifacts that she sold. Ended up with close to 5,000 simleons. Pretty good payback on the cost of the trip.
I haven't had any luck in find insects, but collect seeds is well worth it, some of the have sold for as much as $45, beats finding gems, rock worth only $8-20 cash. In Riverview, at each end of the island, I find quite a few seeds that makes it well worth the quick stop.
Another sim who is an artist delived a painting in China. At the base camp, you can open the bookcases and take all of the books and sell them when you get back home.
New to this but compared to my being an artist, writer and surgeons, this is one that I think will provide the most cash and is interesting.