For what it’s worth (probably not much) as a very unhappy only child my advice to the parents of only children would be:
Help your child to integrate with their peers as much as possible, including helping them to keep in touch with the cultural touch points of their peers - the TV shows they watch, the music they listen to, etc.
Don’t try to live a childfree life with a child in it (agree with the previous poster about organising holidays for adults while dragging a child along - this was my experience too).
Play with your child - don’t expect them to entertain themselves most of the time. (I think in some ways it’s harder work being the parent of an only than two.
Conversely, don’t smother your child and let them be themselves - it can be suffocating being the sole focus of your parents’ attention and expectations.
Finally - and this one’s really hard to articulate and I may fail miserably so bear with me - don’t ‘gang up’ with your child’s other parent against your child. I’ve noticed how in families with more than one child, including my own, the children get together to ask for something; or often if they’re told off, they’re told off together. Even if they bicker all the time, there’s more than one person in the ‘child’ camp. If one sibling wants something they can say ‘you let Jack have it’ etc etc. I just think it’s important to hear in mind that there’s a huge power imbalance between two adults and one child.
The stereotype is of only children being spoilt but I think I was much less spoilt than friends with siblings because I had so much less ‘leverage’ - my parents could maintain a very strict stance on things and my power to challenge them and change their minds was very limited.