My husband and I are in the same position and have a 15 month-old nephew. We've also decided against having kids of our own. My brother-in-law and his wife aren't rich by any means, but they're doing okay financially. They're super frugal and have lots of side hustles. Because of this, we don't feel like we need to give our nephew lots of cash or start savings accounts for him. Instead, our goal is to spend as much time with him as possible, and when the situation warrants, use the money we have to help pay for experiences that he wouldn't necessarily get with his parents. We want to be the "cool" aunt and uncle, not because we buy him things, but because we expose him to a lot. He gets plenty of toys, but without us, he might not learn to fish or backpack or or or...you name it.
One thing I didn't realize is that you can contribute to a 529 for a niece/nephew or child's friend and transfer it to your own kid if you do eventually have one. We're pretty much a "no" there, so it doesn't seem worth it to us. And what if the kid doesn't want to go to college? I'd rather give him a chunk of change later on that can be used for anything if he really needs it (house downpayment, starting a business, etc.)