A really lovely island that hosts a triathlon every year in May. The weather is normally good as before rainy season.
Take a 3 hour Jet ferry from Tokyo to Niijima Saturday morning for a 1pm start (no briefing needed).
Swim: Swim is in a protected beach that has a seawall to stop the swell. A great race for those nervous about the swim as the water is super clear and you can see the bottom the whole way around. Swim is 2 laps.
Bike: The transition is the shortest possible and the bike course is around the island which is very scenic, a few minor slopes, but nothing too tough.
Run: The run starts with a pretty brutal hill which can suck the power out of the legs, but it's a good run course, 2 laps.
There is a free natural onsen very near the finish line where most people go after to clean up and have a few beers, some great sunsets.
Lots of accommodation in town very close to transition/finish and a good after party at the local high school with some good food and drinks (all part of entry free).
Sunday take a cycle tour of the island or go for a surf (on the other side of the island with white sand beach), then catch the fast ferry back to Tokyo at 2:20pm and back in Tokyo by 5:30pm.
One of my favorite races and a must do if you live in Tokyo.