I’m a MIL and have a DIL and SIL but I’m not yet a grandma. I think we have a very good relationship with our children and their spouses and they express similar thoughts.
Basically we treat them like adult friends and don’t make any assumptions. We’ve made it clear that there are no expectations about things and everything is up for discussion if they want to. For example they do what they want at Christmas - we don’t expect them to have to do strict rotas with other ILs or always come to us or anything like that. It seems to work out that way as it happens but every Christmas we say ‘You do whatever you want to do, you’re always welcome here and we are always willing to travel but equally you have other people to consider so we will work around you or will see you in January or whatever.’ I think we have a better relationship because they don’t feel under any obligations so when we see them it’s because they’ve chosen it.
We also don’t give advice unless it’s asked for and make it clear that if we are asked for advice and give it, they’re not obliged to follow it. We respect the way they run their homes and fit in with their house rules. We offer help but always say that it’s fine if they want to do it themselves. For example I’ve helped DIL with some decorating and gardening and helped both couples move house. We always say ‘We’re here and willing if you need us but no worries if you’re sorted.’
Things that my own mum did well
as a grandma was:
Tell me often we were doing a good job as parents.
Ask what would be suitable birthday and Christmas presents before buying something.
Followed the way I wanted things done (according to latest advice) so no ‘Oh it’s fine to feed him rusks at 12 weeks’ or whatever.
Treated all the grandchildren fairly and equally - she still keeps a list of money she’s given for exam successes/weddings etc to make sure she treats the other GC the same!
Had the GC to stay on their own when they were old enough to give us a break and them a fun time. She would research good places to take them in her locality and planned lovely days out for them.
She is the absolute champion of her DC (my DSis and I) her son in laws and her DGC. She’s always on our side and thinks her DGC are the cleverest, most beautiful and loveliest humans to ever walk the planet!!
The very fact that you’re asking this question OP means that you will do a fab job.