https://www.cloudynights.com/topic/499265-narrowband-imaging-targets-for-the-summer/
is a good start - but there were 2 or 3 excellent threads like this one - Google cloudynights narrowband targets or all the key links are at the bottom of the first post
Small excert
M8 Lagoon Nebula (90')
M16 Eagle Nebula (35')
M17 Omega Nebula (46')
NGC 6820 Sh2-86 (40')
M27 Dumbbell Nebula (15')
NGC 6888 Crescent Nebula (20')
IC 5068 (80')
IC 5070 Pelican Nebula (80')
NGC 7000 North American Nebula (150+')
NGC 6960 Western Veil (70')
NGC 6992 Eastern Veil Nebula (60')
IC 1396 Elephant Trunk Nebula (150')
NGC 7293 Helix Nebula (20')
NGC 7380 Wizard Nebula (30')
Sh2-155 Cave Nebula (60') Note: either full narrowband or HaRGB is possible
NGC 7635 Bubble Nebula (40') Note: widefield views can include M52
NGC 7822 (60')
NGC 281 Pacman Nebula (40')