Visa permission is mosgt importantg document for travelling in North Eastern regions of India. While traveling to North-Eastern states like Assam, Meghalaya and Tripura, tourist doesnt require to take special permissions for visits. Possibilities are there that you need to register upon your arrival and departure. Due to strategic geographical location, tourist does need to acquire Restricted Area Permit while entering other North-Eastern states like Arunachal Pradesh, Manipur, Mizoram and Nagaland.
For foreign travelers visiting Sikkim (bordering China), visa permits are readily available free of cost. But, while touring north Sikkim, an Inner Line permission is required, which is issued only to group of four travelers,that too through tour operators.