If you wait until they leave it’s no fun! 
I’m surprised that no one has mentioned this yet, but it’s of extreme importance that you not mix into or criticize your child’s and spouse’s parenting decisions or style. Especially if it’s your son and daughter-in-law, but in any case, less advice is better than more.
If they ask your advice on their own that’s something else. But even there, you need to make sure one parent isn’t trying to get your support in their dispute with the other parent. (And this is especially relevant if it’s your child who is asking your opinion.)
… deeeepends.
I was very, very careful not to criticise Bro and his wife in general, but when I saw for example that they were trying to feed the Nephew twice as much food as his mother ate, I took Bro aside and pointed out that this was absurd and likely to cause unnecessary conflicts with food. And that, since I’m both the Nephew’s aunt (due to randomness) and his Godmother (due to actual parental choice), I do have a duty to point out such mistakes.
On one hand you have to let the new-parents do the parental equivalent of skinning their knees, but on the other sometimes you have a duty to their child to speak up.
You need to be careful about that too, but it’s actually worse coming from a parent than from a sibling. Because there’s more pressure resulting from parental - and parent-in-law - criticism than from someone else’s which can be more casually dismissed.
Yeah well, I thought of mentioning this but didn’t because hey, it’s one of the things that feel like I’m saying it all the time, but I’m the elder sibling who used to play the “motherly” role because the woman with the job title couldn’t be arsed (she hated that Bro and me for decades, too, which didn’t make for a friendly environment). It’s not a normal sibling relationship.
Mom’s attempts at criticism have actually met with “you don’t have a right to speak about that”. There are issues on which my input is accepted and hers isn’t, not because I’m the sib and she’s the parent, but because it was me who used to do whatever parenting task that input is about.
Well, not as much fun. But still secretly fun!
nm
As a grandparent I’ve noticed I have more patience and tolerance than I ever did as a parent. I’m also more likely to be more creative and go with the flow. A couple of weeks ago, my 3 year old granddaughter and I were in the basement for something and she saw a cat carrier down there.
“What’s in there?” she asked, peering down into it.
“That’s where the baby dragon sleeps at night,” I told her. “Is it in there now?”
We spent a good ten minutes looking around the basement for the baby dragon, only to conclude he must have been napping somewhere else.
My eight year old grandson has a spot in the yard that he’s worn down to the bare earth. He builds tunnels and bridges there from sticks and pine cones and whatever he collects around the yard. Speaking of sticks, there’s a box of them in the garage that I’m not allowed to throw out because, well, they’re his sticks that he collected over time and plays with when he’s here. (Yep, he has lots of toys, but apparently large thick sticks make better swords and guns, so whatever.)
I never had hands-on grandparents (separated by distance and strained relationships with their own children), so I’m making this up as I go along.
Grandma and/or Mom will probably handle most diaper duty.
My high school buddy got 5 grandkids in the past 2 years (2 daughters and 1 daughter in law performed the heavy work, and it started off with an adoption) and he loves it.
It’ll be YOUR duty to set up a neat model train layout in your basement to impress your grandson. Remember, he’s got another grandfather. And you’ll have to teach him about blanket forts. And armpit farts and “pull my finger”. As the boy gets older, you’ll be answering a lot of questions. Have fun.