Lachung is a heavenly spot, of north Sikkim. Since you have explored full of lachung here are few more spots you could try exploring further. From lachung 7 hours away is Pelling. Nestled between mountains of west Sikkim here are some things to do
Hee Bermoik is an adventure town
Changey falls is mesmerising
Singshore bridge is a lovely tall bridge
Darap village is a paradise spot
Sewaro garden You could travel to Gangtok capital city of Sikkim Nathula pass is another spot Zuluk is an offbeat spot of Sikkim to explore
Goechala is a mountain pass for trekkers.