Almost everything and all venues are open in both the U.P. and the L.P. until about mid October when the colored leaves start to fall. The many Ferries run to Mackinac Island also and the island stuff is still going. Sure, the crowds are smaller as most families with kids are home as school is open again.
Later in October in the U.P. there can be nights with hard frost or even some freezing in the U.P. so that is why most CG's close by mid October but not all as some are open year around. Even the upper CG at Beautiful Tahquamenon Falls State Park Lower Falls CG which is only ~25 miles south of Lake Superior and is open all year. Water is shut off but electric is on and they plow the roads and sites of snow in winter so there's no snow depth problems. Been there camped several times in January and February before.
Also have RV'ed to the U.P. for over 50 years of our 56 years of marriage in late Sept and Oct for the spectacular colored leaves and then when coming back south to home on the West Michigan Shoreline, we virtually follow the colored leaf changing all the way! Only one trip to the U.P. in the over 50 for fall colors did we have any snow at all and that was about 2" on an October 16th. Usually in the 60's to low 70's in the first 2 weeks of Oct up there. The West side of the state is very different than the east side and The U.P. is very different again. Like 3 distinct different landscapes and features all in one state.
Roughly, only 300,000 people live in the entire Upper Peninsula. About 700,000 live in the northern half on the Lower Peninsula and the remaining 9 million live in the southern half of the Lower Peninsula with approx 5 million of those in the Detroit S.E. Michigan area.
Even later in the L.P. (especially on the west side of the state) as being farther south and warmed by the constant summer heated water rising up from the huge very deep Lake Michigan and then blowing over the land to the east and S.E., more CG's stay open longer as do many venues until after Thanksgiving and several CG's are open all year too. Platte River CG at Sleeping Bear Dunes Lakeshore National Park is open year around and so are the nice tiled and heated bathrooms and private door showers. Gorgeous area!
As said, there's so much to do a see in all of Michigan year around. It sure would help if you'd mention some types of things that interest you as Michigan has so many things to see and/or do.