There are number of Tourist Attractions in Arunachal Pradesh like
Tawang
Sela Pass
Bumla Pass
Bomdila
Itanagar
Tezu
Ziro Valley
Namdapha National Park
Dirang
Pasighat
Golden Pagoda Namsai
You need at least 2 Tours of 10 days each to cover major Attractions of Arunachal Pradesh
Here are some lovely places to explore in Arunachal Pradesh
Sela pass
Itanagar
Bomdila
Aalo
Daporijo
Gorichen peak
Anjaw
Nuranang falls
Mechuka
Some national parks like
Pakhui wildlife sanctuary
Namdapha national park
To cover all places you need 20 days.
Namsai (golden pagoda),
bomdila,
tawang,
dirang,
zero,
parshuram kund, etc
Arunachal Pradesh is spread mostly horizontally. plus it is hilly region so it takes time to travel. that means you will have to travel for longer time and it may take plenty of time. at least 15 days will be required.