You can plan for Darjeeling sight seeing for 2 days where u can visit places like Japanese Pagoda, Rock Garden, Batasia Loop, A ride in a Toy train & Tiger Hills.
you can visit some beautiful village like Gorkhey in Singalila National Park for 2 days.
Rest of the days can be planned to visit Ganktok.
I bet u there is nothing to explore in Darjeeling.The place has become dirty and worthless unlike the old one.Better utilised ur time and days in exploring Sikkim.What you will see in Sikkim is what you will observed in Darjeeling too.Based on my recent experiences.I wasted 3 days in Darjeeling and Kalimpong.
Visit North sikkim with a halt in Gangtok for the evening stay and next morning with shared cabs Pelling , Gurudogmar Lake ,Buddha Park in Ravangla , Tsogmo Lake are some prominent places to visit . Check my youtube channel UnexploredNortheast blogs about Sikkim featured