A 3 hour drive after a flight gives you so many possibilities. Albany airport:

Lake Placid
Saranac
Saratoga Springs

Or you could go south to the Hudson Valley (numerous pleasant towns) but I prefer the big mountains and more plentiful lakes of the Adirondacks.

Thanks, everyone! This is OP, back to answer some questions.

DH and I take our kids on road trips regularly and can drive ~10 hours in a day to our destination. It's harder for our family or four to fly on a budget.

ILs prefer flying and can't really be in a car for more than a 2-3 hour drive.

We're leaning toward renting a house or finding a resort on/near a beach/lake. My MIL will get bored if there aren't other things to do, like a town and hiking trails.

Any suggestions in the Poconos, Michigan shore, or Delaware? Or maybe along Lake Erie or in the Catskills or Adirondacks, but would need to be near enough to an airport for my ILs.

How far are you willing to travel? I grew up in New England and love Western Mass and that’s easy distance-wise. But the West has some easy and fun get aways. I love traveling to Sun Valley, Idaho in the summer. Not that easy to get to, but very easy when you’re there.

Or Rhode Island; go to Newport, Watch Hill.

Western Massachusetts; go to Amherst, Northampton, Williamstown, Great Barrington, Lenox, etc.

Meant to add: Boston and take a ferry over to MV, ride up to RI or out to Cape Cod.