You might really like North sikkim. it is awesome at that time of the year. if you would like I can share some of the pics from my last year trip. I did the Goeach la trek. it's a toughness level 6 to 7 trek. The scenic beauty is absolutely mind blowing.
You can visit Dandeli or Chikmagaluru for Adventure/ Trekking you will get to see lot of hills like baba budangiri, mullayyanagiri,Z point and also you will get to see waterfalls.