Hey.. yeah u can travel solo. I have been to North East with my mother and we two were on a trip to Assam and Meghalaya. Not even for a moment we faced any difficulty. So, it's completely safe to travel solo. But, for your personal satisfaction u can carry pepper spray.
Hey, I believe Sikkim is a spectacular place to travel within India, however if u want to consider any other option within northeast, I would suggest you checkout the marvel of Meghalaya. The main attractions in Meghalaya are as follows: Within Shilong - 1) Elephant falls
2) Don bosco Museum
3) Wards lake
4) The many view points to get a gorgeous view of the Khasi valley from the city Nearby Shillong: 1) Dawki (I'll rate it no. one as this is the place where the water is so clear that the boats appear as floating in the air, checkout some google images to believe me!) 2) Shnongpedong (a lesser known cousin of Dawki, but equally pristine, known for river side camping) 3) Mawlynnong (cleanest village of Asia) 4) Mawsynrum (wettest places on Earth) 5) Cherrapunji (which has a variety of attractions including Swami vivekanand mission, 7 sister falls, Nohkalikai Falls, lush parks and view points) 6) Krang suri falls (I have not visited it yet, but the photos are simply droolworthy!) Hope this helps :)