this is my 6th baby and the best thing i have found which isn't an essential but a luxery was a swing, baby loved being in there i think they like the movement and i could get on with looking after other kids, making tea, do a bit of housework etc. a baby sling also i found really helpful again with having other kids when they needed a drink making etc and baby wanted to be held or breastfed i hand my hands free.
I do use my babymonitors because i put baby to bed from a couple of weeks old upstairs to help them get into a routine of night and day, but i just use the sound ones which i have used for all of them. I still use mine now as baby is only 9 months old and he goes to bed at 6.30pm now i turn it off when we go up to bed.
I do agree a sleeping bag so baby cant kick covers off aswell
i will try and think of some more