Hey! kindly check my blog over sikkim u will get idea for the same.
for 7 days u may go this way
day 1 and 2 gangtok local sightseeing
day 3 east Sikkim which includes Nathula pass, Baba mandir, tsomgo lake
day 4.5.6- North Sikkim which includes lachen, lachung, gurudongmar lake, yumthang valley, zero point.
day 7 Pelling for sky walk
hope this helps.:)