How much airplane ticket you got to hokkaido return?

Depends. If you plan going Sapporo + Otaru + Furano only then take train also can.

If you plan adding Lake Toya + Hakodate + Noboribetsu + then car rental will be the best.

On our two weeks Hokkaido trip, we arranged to pickup rental car on the day we arrived, and immediate drove out of Sapporo. Returned after ~10 days and returned the car. After that, spent few days in Sapporo and a day-trip train to Otaru. Our opinion and plan is we do not need a car in Sapporo city and Otaru. We spend the last few days in Sapporo rather than in the beginning due to not going to carry all the stuff to buy and tour around Hokkaido.
Parking in Sapporo costs $ while most (if not all) outside are free parking. Also less few days car rental cost.

That would be almost all around Hokkaido already. Hakodate is South, Noboribetsu is in central while Biei and Furano is in the East.Unless you have a lot of days there, I would suggest you keep your travel to a few nearby cities and attractions. Car rental would be more suited to if you are travelling to outskirts. Like mentioned, if you plan to go around Sapporo/Otaru and Chitose, then bus/train would be good enough.

Yes. You will be running all around touch and go at most attractions and spend time driving to the next. Suggest you plan around central and south side or central and east side.

hi all, im going to Kyushu in late March for 8D7N. here's my plan, kindly give comments if you have a better idea

Day 1 - touched down Fukuoka and drive straight to Nagasaki. Explore Nagasaki and stay there.
Day 2 - Unzen & Kumamoto. Stays Kumamoto
Day 3 - Miyazaki
Day 4 - Takachiho gorge & mount Aso. Stays Aso
Day 5 - Yufuin & Beppu
Day 6 - Fukuoka
Day 7 - Fukuoka
Day 8 - fly back