I only do tours when there's Giant-Dragon, Spider-Crab, or Tuna Kahuna tours, since those will give me the most amount of medals.
So I get about 20 medals per tour and I can get a lot of tours in just an hour or two.
I like the Giant Dragonfly or Elite Gardener. I prefer the gardener one because I can finish in about 3 minutes on average for 12 medals, and don't have to wait for the 7 minute timer on the dragonfly one to run out.
I can play the Labyrinth tour over and over, so I guess I get a lot of medals for that. I never really buy anything from the island so I have a few hundred medals haha.
The Giant-Dragonfly, Tuna Kahuna, Scavenger Hunt and Ore Hunter tours are my favourite tours to make money off of. For playing with others the Hide and Seek, Hammer and Labyrinth tours are really fun! I like the Ore Hunter tours with others too but to be honest I have trouble keeping track of the secret ore sometimes.
I wanna try Hide n Seek with 4 people because I heard it's a lot of medals but I'm not usually playing with more than one person, and very rarely can I find 4 people on Club Tortimer all willing to play games. :s