When I'd first visited Sevii Islands, I only thought that there were only three of them--One, Two, and Three--and didn't know about the rest. Back then, I wasn't much of an Internet user, so to speak, so I wasn't able to figure out all the rest of islands. One day, I managed to hit certain number (which I can't exactly remember) in my PokéDex, visited Professor Oak, and he eventually told me about these places. Was very excited to visit them, because why wouldn't I be? To me, they were completely new and whatnot.
When I've finally got there, I've spend some decent amount of time exploring them, and did almost all quests that they held. Most fun, to me, was the Team Rocket one, where you got to unlock their secret hideout with some password and defeat their executive (or was it somebody else?).
There might've been a few other missions, but I'm not really sure if I had managed to complete those, unfortunately.