The tuna-kahuna tour is my favorite for earning medals. You just have to catch 8 tuna in 6 minutes, which is pretty easy to do, and you score 20 medals. When it shows up, I usually do it several times.
If I have four players though, the hide-and-seek tour works well (I prefer the official one over the elite one, since the elite one has pitfalls). You only get nine medals for the official one, but it ends as soon as you find all four animals. And with four players, this can often happen within a minute or two. So it's a good choice for large groups.
Depending on your skills I like doing the shark tour and the scavenger, I find them the easiest and I just keep doing them over and over so I have a bunch saved for when I need something off the island.
The tuna-kahuna tour is my favorite for earning medals. You just have to catch 8 tuna in 6 minutes, which is pretty easy to do, and you score 20 medals. When it shows up, I usually do it several times.
The medals you get from tours, I believe. Anyways, I've found the Tuna Kahuna Tour to be pretty good (20 medals for Gold) as long as you're good at catching fish. But Elite Gardening Tour nets you easy medals so it's up to you.
Only take the tours you like, that way it doesn't seem like such a chore. My favorite is Hide-and-Seek. Doesn't give many medals but time flies because it's fun!