"Rebun island" and "Akan National Park" - specifically Lakes Kussharo and Mashu - for Sapporo.
I think you are terribly confused about the geography here. Sapporo is a city on the island of
Hokkaido. It is nowhere near Rebun or Akan. To get to either of those places, yes, you will have to come to Hokkaido, but Rebun is many hours to the north and a 2-hr ferry ride from Wakkanai. Akan is on the other side of Hokkaido, several hours away by train.
Those are very nice places to visit, but were you just thinking Hokkaido and wrote Sapporo by mistake? I hope so.
You could easily burn a week doing just those places, mostly because of the travel time involved on Hokkaido. If your plans fall through, or you have extra time, I'd suggest a couple more spots. You'd likely have time to visit only one of them, so judge according to your own interests.
1) Otaru, about 45 minutes north of Sapporo by train. There is a canal, glass craft shops, woodcraft shops, a glassblowing exhibition shop (where you can pay to take part if you like), and sushi street which is wall to wall sushi restaurants!
2) Lake Toya, where you can walk through a smoldering volcano next to the town itself. The volcano erupted in 2000 and covered the town in ash. Nearby, you can also visit a much older smoldering volcano if you like (and have the means), called Showa Shinzan. In any case, get a room at a hotel and enjoy the onsen inside.
3) Marukoma Onsen (or other onsens) on Lake Shikotsu would also be nice. MO is the most expensive one on the lake. Further around the lake edge is a cheaper one, and it is there that you can see summer fireworks at night.
4) Sapporo Beer Factory has a genghis khan (lamb) luncheon. Sapporo has a city bus tour. The huge Odori Park has events sometimes (flower show and beer garden in summer).
5) Noboribetsu to the south of Sapporo has a huge sulfur pit. Avoid the bear park there.
6) Hakodate is famous for its night view from Mt. Hakodate and for its seafood.
7) Matsumae has the island's only castle (rebuilt), and although this is nice enough, you can see better elsewhere. The best time to visit Matsumae is spring anyway for the plethora of cherry blossoms.
8) Furano has huge fields of lavender which peak in late July.
Also, what time of year were you planning to visit?