During the summer months of June to August, Rebunto burst into bloom and that's where visitors start flocking to the island. The island hosts some alpine flowers which are supposedly found nowhere else! There are also a good selection of hiking options, as well as acommodation options on the island, all of which can be found right in the ferry terminal (most is in Japanese, but there are some English information to get by).
Unfortunately, I went on a hiking trail (the 4-hour course) that features more of the coastal scenery than the flowers, so I did not get to see many of the famed flora. Nevertheless, it was a great walk with great views, and I really enjoyed it. And the trail ended right at the northern tip of the island, where there is a minshuku (a homestay) which claims to be the northernmost minshuku in Japan! (no, I didn't stay there)
Access to Rebunto (and Rishirito) is via Heartland Ferry from Wakkanai. There are limited express trains and buses to Wakkanai from Sapporo. It is possible to get from Sapporo to Rebunto in a day, but you have to get the earliest train noting the ferry schedule. If you are a hiking and flower fan visiting Hokkaido in Japan, do check out Rebun island!